Depends on where you live and what you want to do with the bike. For example, if you are a CA resident and you want to ride the bike on state land in CA, you will be outta luck if you don't have the paperwork straight. They could impound the bike if they catch you riding. But if you are a resident from a different state that doesn't title bikes, then you will have no problem regardless of where you travel to for riding.
Here's a good example (it also lists which states have no titling for dirtbikes):
http://ohv.parks.ca.gov/?page_id=26294#visitor
If you're buying an untitled bike, the world is not ending and it probably isn't stolen... Many states don't title dirtbikes. Anyway, if you're worried about it, call the police station and ask them to run the VIN. That plus a bill of sale is all that's done in many states. If you need to title it to use it, that could be a hassle (depending on your specific situation). But if you don't ride state land (you ride mx tracks instead) or if you live in a different state that doesn't title, then who cares...