The type of wood you are using matters too. With MDF I have never sanded a thing, just roller painted it, and it always came out just fine.
I did a Defender cabinet once with sandpaper and automotive spray paint cans and it came out absolutely fabulous, but that was like 20 cans of paint and more coats than I can even count. I'll never do it that way again, I must have put 20+ hours just into painting that sucker.