I use gowns at night until they stop pooping at night (around 4-6 weeks) Like the PP said, it makes night time changes easier.
I do dress my babies every day in outfits, but they are mostly one piece outfits. Most of my 3 month size outfits are actually onesies, but ones that have designs/patterns on them. My babies are into 6 month size clothing by around 6 weeks, though.
I never use those plain white undershirt style onesies. I live where it's hot, and I've had 3 summer babies. Just no need for those.