A friend and I debated this one a few weeks ago. One thing I've noticed is much better QC procedures from the CPU mfg's.
The outer shell of the newer CPU's (AMD) are much straighter from my experience in my last 4 builds than they've ever been before. The added excellent cooling features from companies like Thermaltake, CoolerMaster and Corsair make it almost unnecessary IMO to bother with lapping anymore unless your planning on using a stock cooler.
That's not to say you won't benefit from a 1-3 C temperature drop from lapping but with the cooling becoming so much more efficient it's not needed like it was 3-5 years ago.
Every little bit helps so I wouldn't say "don't do it" but if you're going with something like a Hyper 212, H50/H70, or the new FRIO I don't know that it's really worth the time invested anymore.