I assume this is because it'll be a used MBP? If you come to any screen but the welcome screen, then just pop in the system restore discs and erase and restore the computer to its original software. Create your user (the system will guide you through it), log in, and update from there.
Note: if this MBP didn't originally come with Snow Leopard (earlier than the mid-2010 refresh), and it has it on it now, my method will simply bring it back to Leopard (or Tiger, if it's early enough). In that case, you'll want to make sure you get the SL disc from whoever you're buying it from.
If, OTOH, you are buying it new, you will be the first to boot it (as far as the system is concerned at least; it was, of course, test booted at least once before being packaged up).