I use both the harness and the collar on Chloey and Naiya. The collar has their tags on them and also a little bell so i know where they are at all times. When we go for walks i just slip the harness on and we're off!
The only time i really take the collar off is at night because they sleep in bed with me and hearing a bell tinkle every time they move is enough to make me go crazy!
P.S. Both my doggies have breakaway collars on them. I find it's really the safest bet.