Years ago when the quality of the CPU caps were definitely in question (surface variances) this did matter. Reduction in temps of 8-10C though is not realistic for most users today (2012).
Also, the reasons for doing this were born from the fact we had to use stock heatsinks which were woefully inadequate of handling any extreme changes in processor temperatures due to overclocking. 80-92mm heatsink fans were enough to handle stock settings but not much more than that. Since then, the quality of the caps are much higher than they ever were AND we have much better choices now in cooling the CPU that were unheard of just a few years ago.
Watercooling, sealed watercooling units, extreme pipelines, push/pull stand up heatsinks, 120mm fans (single and or dual config) these have all made it practically useless to bother with lapping anymore. If at best you're going to get a "probable" 1-3 degree C temp change, is it worth 2-3 hours or more of work? And almost all users who lap don't even start with a known setpoint so they can't even tell what, if any gains, they've actually made. These are not assumptions they're known facts.
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Not knowing what "Lapping" was, I found this great explanation: http://hubpages.com/hub/How-to-destroy-your-CPU-by-lapping-it
– Greg Hewgill – 2009-07-21T01:49:56.043