here's my experience....
i had a nice spare road crank, probably sugino or SR (sakae) and a 97 gary fisher MTB. i thought i would switch cranks. seeing as how the MTB had a freehub, and i had a variety of spacers, i thought that chainline wouldn't be an issue. and as i turned out, it wasn't.
first off i should say that i did not replace the BB, at first.
i removed the old crank and put the drive side crank on. there was no chainring attached. the spider (sans chainring) would not clear the chainstay
. so i put on a different BB with a longer spindle. it cleared. yeah
so i put on a chainring (36 teeth, i believe) and it wouldn't clear the chainstay
, so i moved it to the outside (large chainring position). and it cleared. yeah
then i put on the non drive side crank arm and it wouldn't clear the chainstay
. i was out of BB's so i gave up. geometry's different. learned a lesson.