It doesn't hurt anything at all, and it is sometimes necessary for keeping the knees from contributing to the press. I can't imagine it being a problem.
During standing exercises, I often find myself flexing the quads very hard, kind of pushing upward with them, and squeezing my glutes in in order to assist with the movement (especially lockout). I'm mainly talking about the Press, but have done the same thing with things like curls. Is there any reason I should not be doing this? I sometimes get the feeling that I am hyperextending the knee (not sure if thats the right word). I have also heard that flexing the quads is correct from some PLers and such, but I did not see any mention of this in SS.
It doesn't hurt anything at all, and it is sometimes necessary for keeping the knees from contributing to the press. I can't imagine it being a problem.
It's a good thing to have a lot of tension in a standing press. It gives you a strong base to press against. Keep doing it.