Well, if you hit yourself, then I don't think you'd be using the fullest extent of your power because your mind may tell you it will hurt. But if you hit someone else, then you're not getting hurt. You're punching to the fullest extent of your power, and in turn, hurting the other person you're targeting.
I think it depends where you're hitting yourself as well. For example, pulling a Peter Coffin and kicking yourself in the balls. Getting hit in the balls hurts like 20 motherfuckers. So would that mean you're weak or strong? Well, I think if you destroy your testicles, then you not only gotta be strong, but you must have a very, very high pain threshold.
Now if you hit yourself in the leg, I could imagine somewhat going for some power. But would it hurt? I dunno. Everyone's got different thresholds for pain as well. For me personally, I punch with my full power, my leg is gonna brace for impact and it won't hurt as bad.
I guess in the end, it comes down to the individual and his/her threshold for pain. I don't think you can be right or wrong in this case.