I will have to say that those are some of the best temps that I have seen reported in these forums for running Prime95 on a 3750 at 4.5ghz. Very good temps at idle and there is one thing to remember and that is when you run a stress test like Prime95 , the temps that you get will be the highest that you will see because in real world use it is very hard to get 100% load so that's why I said that those will be the highest temps that you will see.
No there is not such a thing as not enough voltage and it's actually the opposite in that when overclocking you should only add enough voltage to make the overclock stable. So if you can get your overclock to be 100% stable with the voltage that you have then it's a win , win situation for you. The more voltage that you add the more heat that your cpu will produce and the biggest enemy of the cpu is heat. If your 100% stable with your settings then by all means you should keep it where you have it.