It depends what you stress your rig with. If you are OC'ing for games it depends on whether the games are CPU or GPU intensive. You are likely to see higher temps on the CPU with something like Flight Simulator than with Crysis II, BF3, etc.
You should be concerned with temps exceeding 60 C for a prolonged period. Really critical heat starts 80 C.
Running Prime 95 will probably get you above 70 C. It is not cause for concern as Prime 95 is an artifical stress test and does not indicate normal operation.