If you come up with a nice riff, but something about it sounds familiar, is it best to start over?

Just take the part of the riff that sounds the most familiar and tweak it. You could change the phrasing or the timing of that part of the riff or substitute one note for another. Replace a couple of notes with their octaves or through in a ping note in between some of the notes. Keep tinkering with it, until it feels like your own, and if someone still says that it sounds like Sweet Child of Mine, then scrap it.