Ginger and Bernie wear their collars with tags ALL THE TIME. My theory is, that when a little one goes missing/runs away its not like you can stop them and say "wait, I need to put your collar on you first!"
They wear harnesses to go walking.
Now, for a long time I tried to coordinate harnesses/collars. When I just had Ginger I even purchase multiple i.d. tags. This was a pain, was not cost effective, and was a pain!
So...they both have thin, nondescript, plain, black collars with their tags. These coordinate with all of their harnesses and life is much easier now.