Not a big issue, I've self-served everything from c150's to a TBM850(Jet-A sucks self-serve, bring latex gloves!!) Just remember it's not a race, take your time, and you'll be fine. There's really no reason to have the fuel filled all the way to the rim of the tank, leave at least 1/2-1" below the filler.

If you're on an uneven surface(I've seen 5 degree's once) in a cessna, close your fuel valve before filling, the cross-fill will transfer fuel right out of the plane while you're doing something and not paying attention. **Disclaimer**Of course don't forget to turn it on after you get back in. The plane will run just long enough to get to the runway in most situations, so it can catch you off guard. Also be careful of supplied ladders at the self-serve farms, some of them are nice, some will kill you!