[Murray]

Yes, tiller is the difference in angle of the top limb and the bottom one. Measured as the difference in distance between the base of the top limb (near the limb pocket) and the string (measured at 90 degrees from the string), and the same measurement on the bottom limb.

It's generally touted as a good idea for two main reasons:
1. The throat of the grip is not in the middle of the riser
2. We shoot one finger above and two below

Somewhere between 0mm and +4mm is usual (+4mm = string 4mm further away from the top limb).

There are many ways to check for good tiller - drawing the bow back in a straight line and watching to ensure the longrod doesn't rise or fall is one.