All of the bands I've been in (bar one, which was hard rock from the outset) have been a "let's see what happens" sort of thing...
One band had a jazz-funk drummer, punk mohican singer, blues keyboardist, heavy metal guitarist and me (into anything groovy and out-there); we produced a sort of James Chance and the Contortions meet Butthole Surfers noise...
Current band has a hardcore punk singer, classic old-skool punk bass player, me as other bass player-come-widdle-fest-merchant, a guitarist into psychedelia and Hendrix and 70s rock inspired drummer. As a result we don't really sound like any of the other bands on the punk circuit; we have 6 minute songs based on an Ornette Coleman riff, 1 minute 50second punk bangers with slap bass solo, slow chuggy grunge numbers with a tango based chorus, phrygian jazzy middle 8 and double speed rockabilly ending...
So I reckon that if you decide in advance exactly what you'll sound like, that's what you're likely to do but you're less likely to produce anything off-the-wall and ever so slightly crazed...