get JB weld and put some in the hex head then put your wrench in and put a little more around it to make sure its going to hold well. Let it set overnight and take the screw loose in the morning. Good tip I read in a magazine and its the only way to take out stripped screws holding the head onto your motor.

Don't get ball tiped wrenchs either. They don't give you a solid fit because the surface area thats in the screw head isn't as big as a non ball tip allen driver. If you do have the money its not a bad idea to get a set of ball tipped wrenchs because there are some screws you can only get at on an angle but try to avoid using them.