So I did some informal testing, with probably a large margin of error, I have some pictographical results.
I picked Blitzcrank, because I own him(it?) and he has a base 300 ms. 8 Seconds of walking down the bottom lane put me a little ways behind of the inhibitor.
Purchasing boots of speed boosted him up to 350 ms, and 8 seconds brought him past the inhibitor, put not yet past the inhibitor turret.
I attempted to record results for 406 and 455 (like I said, it's hard to get the higher numbers precisely) but botched the results, and in each case I was only about the same spot as 350 ms.
Finally, I bought Phantom Dancer and Boots 5, hit ghost (un-talented, sadly) and activated rank-5 overdrive to see how far I could get when speed was pushed to egregious levels. Since Overdrive lasts for 8 seconds, it made for a far more accurate timestop. I made it here.
So we can safely conclude that, unless you're backed in to your own base, it is always faster to recall.
Now if you'll excuse me, I'm going to go avenge my horrendous 6 - 4 previous test game. I will get Ashe Bot this time. grumble grumble
After many more attempts, I have come up with the placement (at the Red buff), but I do not yet have a screenshot to back it up. I will attempt to explain it as best I can, and will try to get a screenshot at some point, to update the answer.
The biggest issue I was having was that I would throw the axe, hit all 3, but not be able to pick it up to lower the cooldown, without moving. What I've found however, is if you stand behind Red, between Red and the smaller vertically aligned creep, as close to red as possible without attacking, you can throw the axe to the other side of Red (so that your cursor is the red attack sword, but it is on the opposite side of Red.)
It may take a bit of experimentation, but this seems to hit all 3, and causes a reduced cooldown. The axe has a larger radius of effect than I expected, so you do not need to place the cursor all the way on the far creep.
Hopefully this helps someone. If no one else, it helped me.
Best Answer
Rammus' great strength comes with the element of surprise. If Rammus isn't in your face, and he isn't powerballing, then he's not scary.
First, the general Powerball gank-tip: start charging it before you actually go in to gank. It takes some time to get spinning, but lasts for quite awhile. If you just press Q and charge on in, you give your opponent precious escape-time. Figuring out the timing on this is critical.
Second, wards. Namely, destroying their's. Rammus late-game can be something like Fiddlesticks; if you can get the jump on them from a surprising position, you've really got something. Have an oracle's on your team (either you or your support) and clear out those wards. Leave them guessing.
Third, scaling MR blues. Rammus needs MR, and these provide a healthy chunk.
Fourth, effective team communication. When Rammus goes in, he goes in. He isn't getting back out. If your team is not prepared to jump in after you, you're going to have a bad time.
Fifth, make use of your early-game advantage. Rammus is FRIGHTENING during the laning phase. If you manage your Powerball correctly, and your lanemate has any CC at all, you're pretty much guaranteed a kill. Gank frequently.
Sixth, give your lanemates the kills, whenever and wherever possible. Don't ever let a kill get away, but if the opponent is certain to drop, give it to your lanemate. They can put that extra gold to much better use.