I go on periods. sometimes all my basses are strung with flats, sometimes I pick one and string it with rounds and use it most of the time, and other times I only keep rounds on every bass.
it depends on what I play at home, mostly, because live it's hard to really tell the difference in the bands I play in, due to the kind of music.
for example, I don't slap very much, and play mostly in pop, rock and blues bands, so flats and rounds sound almost the same in the overall context. I don't play with "agressive pick style" so that I need overly bright sounds, and again, flats or rounds are the same.
so I "decide" at home; if I'm in the "slap" mood and playing at home is mostly slapping and popping, I switch to rounds. if I'm in the "muted motown vibe", and want the feeling of flats, they come back on.
the nice thing is that flats don't die, so I can dismount them and keep the sets in my drawer, waiting to be used.
when I play in studio, it's a different thing, and it depends on the song.