The wheels that come in the BSA kits are not all the same. They look alike, but they’re created using different molds. Some molds aren’t very round, some have crooked axle holes, and some have smaller holes than others.
Crooked axle bores are no good. With all that work you’ve done to align your wheels, you don’t want to install crooked wheels. And if you want the extra speed edge, using a matched set of wheels will ensure that all your wheels spin the same way. Maximum Velocity has a chart of the different wheel molds and their characteristics.
Your car will have a little extra edge if you pick a matched set of wheels. You can buy BSA wheels in packs of five from Scouting supply stores for just over a dollar. Buy several boxes and pick yourself a matched set. You can also buy a set of already matched wheels directly from Maximum Velocity.