It's never a waste of money to have it custom mapped, however, if you don't feel any differences in your bike, then no need to spend the extra $$$. Custom tuning is really for when you can notice a difference somewhere...for example, sluggish at a certain point, etc, or when you install a full system. Also, with just a slip on, a PC really won't make that much of a difference. It definitely helps and keeps your bike from running too lean, but it's not a requirement to have. The map that they probably gave you is a general map for that pipe and easily downloadable off of dynojet's site.
Custom mapping is definitely cool to watch, because you can see how it really can make a difference in getting a smoother curve...but with just a slip on, you really don't need it. Save your money.