Naptha is a great, all-purpose cleaner that's highly flamable. Whenever you paint a guitar the last thing you do before you shoot it with a sealer or base coat or color is wipe it down with naptha. It cuts the oils from your fingers left behind and it evaporates very quickly. Naptha is also not a solvent used for any finishes, that I'm aware of. Another material a bit safer than naptha, but still dangerous, is denatured alcohol. Again, good cleasing properties and evaporates quickly. While it's flamable too, it's not nearly as violent in it's combustion as naptha.
I'd suggest a commercial guitar cleaner/polish kit for basic stuff. I've used cleaners like 409, Windex, etc w/o any harm but I knew the finishes were strong and in good shape. Most commercial finishes today are polyurethane and anything other than acid or lacquer remover should be ok, but always test first.
