Only downside is if you fill it with the wrong grade/brand. Won't matter for most, but some people really get bent out of shape by that.
Topping off fluids, changing air filter, etc. the basic service stuff, that would be fine.
I recently took a class about making customers happy. The instructor told us a story about a hotel he stayed at in Yuma, AZ while driving from Phoenix to San Diego one summer. It tuns out that Yuma has bugs the size of VWs, so by the time he got to this little Holiday Inn, his windshield was covered in bug carcasses. He checked in, went to his room, and after settling in, looked out the window. Down below, there was a teenager the hotel had hired cleaning the windshield of every car in the hotel parking lot. It was a very simple item, but somebody at that hotel noticed a problem all their clients had and fixed it. It wasn't something custoemrs asked for, but most probably noticed. Those littel differences are what set you apart from all the other guys.