As a sign company we do it all the time (incl mine) and it really is very easy to apply 3mm straight cut vinyl to the rims.
Just make sure you clean the wheels properly 1st, take the wheels off the bike, take your time, and overlap the joints by a couple of mm.
If you were to take a close look at any professional race bikes graphics (including the wheels on toselands F05),they are all using this method.
Painting them is fiddly,hence expensive.
