There are folks here who say that you shouldn't caulk between the surround base and the tub because it seals any moisture in. Apparently there is a lip on the tub that goes up behind the shower surround, so no moisture actually gets to the wall.
Some also say not to caulk around the top of the shower surround where it meets the wall, because it has to "breathe" back there. But I did caulk up there because I'm rather tall and water does splash on the wall and I didn't want it running down between the wall and the surround.