In this day and age, using a condom is more important than ever.
Sure, there are those pesky STDs and all that, but the risk of an unwanted pregnancy looms ever larger. And it may only be unwanted by you, the hobbyist. A provider offering you the bare back experience may actually want to conceive and carry to term.
Even if she doesn't want to involve you, even if you never see her again, you could be involved later on inadvertently.
Suppose it is now years later. You are on a positive life path with a good job, loving wife and kids going to soccer etc . . .and you are contacted by some young adult looking for their father. You can stammer and deny all you want, but if you have ever used "Ancestry.com" or some other genealogy website, you are stuck with a lot of explaining to do.
Now, it could all work out well. If I were writing the screen play for a rom-com it would all be fine in the end. But, how many ways could it end badly in real life?
USE A CONDOM!