[quote user="mmolnar"]Do they compact better because of the water in the soil after a rain?
Matt
[/quote]
Yes. Rolling is best done when the ground is soft. Basically, the softer the better so long as the rolling equipment doesn't get stuck or dig in and do damage. If the ground is too hard, the roller just bounces over it and does nothing.
When I do it, it looks like I'm running a squeegy over it. There's a big puddle being squeezed out and runs in front of the roller.
The other thing when rolling is to go slow. The objective is to knock the high spots down. If you go over it too fast, it doesn't have time to fully flatten the bump. Think of a cooking roller going over some dough. If you only get one pass over it, if you did it real slow, it'll flatten it a lot more than going fast.