depends what sort of processor you refer to ? If its a PIV there is no parts of the processor that do not sit flush with the heatsink. An AMD CPU however has much more surface area that doesn't contact the heatsink than does !
But all that pointlessness aside I personally would use the thermal diode on the back of your Graphics card, as ambient case, and CPU temps should all be available via the BIOS and a program like Motherboard Monitor in windows. All in all its not that useful unless there is a piece of hardware you are stressing specifically, and you would like to keep trace of whether or not you are overworking it !
Tim
<font color=blue>Its winter now... So how come my CPU temp is still </font color=blue><b><font color=red>55C</font color=red></b>