Quote:
Originally Posted by biff  High end coolers do usually have a good finish but that not always guaranteed. Check it by holding a razor to it and hold it up to a light to look for any light creeping under to check for flatness. The IHS on cpu's is another story, they can be pretty far from flat. |
+1. Its usually the CPU that needs lapping far more than the HSF.
Found a nice pic to demonstrate (google image search = win!):
See how the edges after step 3 (lower left pic) are still untouched after the lapping has already started? this means that the center of the CPU had a "high spot" as compared to the corners. Lapping will increase the contact area and reduce the amount of TIM you need between it and your HSF.
slightly OT: I like how this user drew an "X" on the HSF to be able to easily spot the high/low spots. Cool.