Nokia's stock kernel is quite generous with voltage as well.

I use only 1.215v for 900mhz, but the stock kernel is 1.350v for 600mhz.

If you are undervolting AND overclocking, it greatly mitigates any possible heat increase anyways.

Also remember, we're dealing with heat dissipation of less than 1 watt for the system-on-a-chip's power use. The wireless chip will generate more heat than the CPU.