So I can't speak for other schools but I recently attended an informational webinar for UIWRSO and they mentioned that you are welcome to take the GRE, but keep in mind that since the GRE does not focus on science, they will place more weight on your science grades during school. I guess if you did poorly in your Physics class during undergrad, that can be buffered by a good OAT physics score. However, when you take the GRE, there is not really a buffer for poor grades. Hope that makes sense and once again, this is only from UIWRSO. If you're not concerned about this aspect, then I don't think taking the GRE will look bad on you.