If we look at low level Rogue abilities such as: Sinister Strike, Backstab, Ambush, etc. We see a consistent trend of a percentage of Weapon Damage. Damage for these abilities is calculated:
base weapon damage + constant * (attack power)/14
In the case of daggers this constant is 1.7 and other one handed weapons 2.4. This is known as AP normalization. Additionally, only the main hand weapon is used in these calculations. As a result it is ideal to use a slower main hand weapon until you have sufficient attack power to influence your choice.
In the case of Rogues it is common to use Daggers as certain abilities (such as Backstab) require them. At latter levels you will gain other abilities which influence this selection (Combat Swords, Mutilate, Hemorrhage, etc), but at lower levels, slow Main Hand daggers is a preferred choice. A longer discussion of Main Hand weapon choices can be found here.
As a subtlety Rogue your "bread and butter" ability is Hemorrhage which does 110% MH damage (159.5% if its a Dagger as Daggers tend to be faster). I would suggest Initiative + Improve Ambush, making your combat look as follows:
- Ambush (or cheap shot)
- Gouge (later you'll use Kidney shot later in the order).
- Backstab (or Hemorrhage)
- Finishing Move
- Hemo -> Repeat till Gouge/Cheapshot come back up or you have the Combo Points to Finishing move
Garrote can be a good opener if the target stays alive for the full duration. As most mobs won't while leveling, Ambush is usually the better choice.
As a Subtly Rogue you also trade on Dodge, Ap, Crit and Armor pen (the last of which you won't get in bulk till much later). Keep in mind that Agility gives you 3/4, but is not as cost effective as straight AP due to Deadliness.
Source
So, there are a few reasons to stand behind a mob, the first three being generally useful to all melee, and the fourth being important specifically to Rogues (and Feral Druids, but they're just wannabe Rogues anyway.)
When you stand behind your target, you do more damage.
Standing behind a melee target negates that enemies chance to parry and block your attacks. In PVP, it negates their chance to dodge as well, but NPC creatures can dodge attacks from behind. Parried attacks do no damage, and Blocked attacks both do less damage, and are unable to crit. If you want to do the most damage possible, stand behind your target.
When you stand behind your target, you take less damage.
This rule is a bit less hard and fast than rule one, but a great many (heck, I'd go so far as to say most) PVE mobs in dungeons and raids feature one or more of the following abilities: Cleave, Breath, Shockwave, etc. The thing they all have in common is that they are targeted in a cone in front of their caster. That means, by standing behind your target, you are safe from these effects. While there are some encounters in which this logic is inverted, or where special mechanics demand that you stand in range of these effects, those are exceptions that you'll learn about on a case by case basis. For the most part, the area in front of a boss is analogous to what most PVE players refer to as The Fire. Don't stand in it. In PvP, players are, for the most part, completely unable to attack you if you are out of their line of sight - read: behind them. If you want to take the least damage possible, stay behind your target
When you stand behind your target, your tank takes less damage.
In general, when an NPC successfully parries an attack, their swing timer (the time before their next attack) is reduced by somewhere in the ballpark of .5 seconds. As a Rogue, if you stand in front of a boss, your attacks will be parried a lot. This will have the effect of making the boss swing substantially faster, causing the nice person who is tanking this boss for you to take substantially more damage. Often, this leads to a scenario referred to as a 'Parry Gib,' wherein a tank takes unhealable amounts of damage in a very short period of time because of a high frequency of parries. As a Rogue, you attack very very fast, and boss mobs have a high chance to parry. In PVE, If you want your tank to take the least damage possible, stay behind your target.
And finally, this last note is specific to Rogues:
When you stand behind your target, you gain access to additional abilities.
As a Rogue, you have three very important skills that can only be used if you are standing behind your target. Backstab, Ambush, and Garrote. In PVE, Subtlety Rogues use Backstab as their primary attack, and Ambush as often as possible (During Shadow Dance, whenever Vanish is available off CD, to open the fight). Assassination Rogues use Backstab instead of Mutilate whenever their target is below 35% Health. Combat Rogues do not use Backstab. In PVP, Subtlety Rogues use Backstab as a primary attack, and all rogues use Ambush to open with a large burst of damage, or Garrote to open with Damage over time and a Silence effect (particularly devastating against mages). If you want to use Backstab, Ambush, or Garrote, stand behind your target.
Best Answer
So, there's a few benefits to using Hemorrhage.
First off, it boosts the damage of your bleed effect - specifically, of Rupture and Garrote. Mainly Rupture is the important one here, because it's not an opener, and because, in fact, it's the highest damage/energy finisher you have available. Especially because, thanks to Sanguinary Vein, you'll always want to have a bleed effect up on your target in order to do more damage with all of your other abilities.
Thing is, the debuff from Hemo doesn't just increase the damage from your bleeds. It increases the damage of bleed effects applied by everyone else in the raid. That means Marksmanship Hunters, Warriors, other Rogues, and especially Feral Druids all will do a lot more damage in groups with you if you keep the Hemo debuff up. Hemorrhage is the Subtlety Rogues contribution to 'raid utility'.
Lastly, and most importantly for solo play, 5 mans, and leveling, is that with the Glyph of Hemorrhage, you can actually use Hemo to apply the bleed necessary to keep Sanguinary Vein active without worrying about using Rupture. This means that by using Hemo as your first unstealthed attack, the damage of all of your other abilities, including auto-attacks and poisons, goes up by 10% for 24 seconds - enough time to kill a short lived mob, and enough time to get a Rupture up on a boss that will live long enough for it to tick out.