I put the bike up on stands and took each wheel off to stripe it.
I've seen rims that were striped while attached to the bike, and the stripes end up looking more like decagons than circles... :eyebrow:
Yes, its a bit more of a process to take off the wheels -- but to get a nearly perfect circle, its the best way to go.
(it's also easiest way to wash all the road crap off of the wheels before putting on the stripes)