I have done it both ways but 95% of the time I cut it after painting. The painting process usually ends with a backing coat. Usually white or silver. If you cut it before painting you end up with a edge thats the same color as the backing. This can really stand out and is not desireable. If you are doing a on road car it is wise to cut the rear wheel well before you paint. Most aftermarket bodies (pro line especially) dont mark those lines so you can get an exact match to your wheelbase. Also get a pair of duratraxx lexan body scissors. They rule Period...... other brands just arent the same. Im not a duratraxx fan but the scissors are the best design available