Well, I guess it's the same idea that we see with a lot of games where we see nVidia logos, Unreal logos, etc.. when they first start-up. You're not claiming the logo or the RPG Maker software, but if you put a "Developed with RPG Maker" along with the logo (or however it's worded so people know you're giving a nod to the RPG Maker software and not trying to claim it as your own), and you're also in a way helping to promote the RPG Maker software (people might think, "Wow! This game was made with RPG Maker? I should get it and try to make a game too."), then I can't see it being such a big deal. But of course, when in doubt, and until the official word is given that it's ok, it's best not to use it.