One of the suppliers in the "where to find" sticky, Jefferson Armory, has a blurb about dished wheels in their cannon history section. In a nut shell, dished wheels make a difference when turning corners. However plain wheels will work just fine maneuvering a cannon out at the range and when under recoil from firing. Dished wheels sure do look nice, but are more effort to build than plain wheels (and more money to purchase if you go that route). As long as you are not representing your cannon as an exact replica, I wouldn't worry about any "wheel cops".
Just make sure to include a power can when you post pictures of your piece. :-)