Path of Exile – Does It Matter What Beasts Are Offered on the Blood Altar?

From the mechanics thread:

Increased Item Rarity Increased Item Rarity % modifiers increase the chances of an item being magic, rare, or unique. For example with a total of +100% increased item rarity, you'd get twice as many magic items, twice as many rares and twice as many uniques from normal enemies. This modifier has no effect on the number or type of currency items, scrolls, or gems that drop. When in a party, only the modifier from the player who lands the killing blow on an enemy is counted. If one of your minions gets a kill, the minion's IIR is added to yours and the total is used.

Magic, rare, and unique monsters have an Increased item rarity modifier for drops.

In most cases it does not matter which weapon is in which hand, although you have to keep in mind that only one will be used in most attacks (except for skill gems that explicitly use both weapons at once of course).

In regard to Main-Hand/Off-Hand calculations there is no such thing as an Off-Hand handicap or anything like that.

According to the game wiki, weapon usage is alternated with most skills. But since only one weapon will be used in most attacks, only that weapons on-hit modifiers will be applied. This might be relevant when you heavily rely on modifers like "life gain on hit" or mana leech, especially when using one of the few skills that explicitly only use the main hand weapon for attacks.