There are many factor's involved in lapping. First, not all Pentium, or AMD IHS (Integrated Heat Spreader) are perfectly flat. In fact a good portion of the time the IHS will be convex. This is done primarily to defend against Heat sinks with imperfect bases (concave), this way where the core is centered under the IHS, contact is ensured. As you may or may not know, between two surfaces only a very small amount will make contact (even perfectly flat surfaces). This is due to the microscopic striations, pores, and pits inherent to many metals.
This page from Koolance is an excellent source of info.
The empirical evidence for my claim, is Thermal Paste. This exists to ensure there's a "seal" or more accurately, medium between the two surfaces in question through which the kinetic energy (heat) will be conducted. If you want to Lap, then you should be using a mirror, or piece of glass upon which to place the grit paper. Then using water or as I use, isopropyl alcohol 70% (which still ensures there's water, but cleans away the metal as well) you would place the heatsink onto the paper, and move the heat sink in a figure eight. This is difficult for me to do rapidly, so I simply go in small circles, reversing direction. I would do this until each grain of paper seems to have exhausted it's use, meaning it's not removing anymore material.
A mirror image is not THE sign of a flat surface, simply a polished surface, this shouldn't necessarily be your goal. Flat is the goal. However; getting a mirrored surface does mean you've done a decent job.
You may want to lap your processor. I have done this twice, and it reduced temps a few celsuis degrees. But remember lapping isn't going to guarantee a large reduction in temp. Changing your cooling method is the best way to do that. If you lap the processor, remember your loosing the warranty, if it's OEM you've nothing to loose. Perhaps the most drastic reduction in temp, will result from removing your IHS. I wouldn't recommend this unless your watercooling, because better air-coolers use stand-offs due to their weight. Northwood's manufactured prior to 11/03 and A64's are safe. However Northwoods and Prescott's manufactured after 11/03 use thermal epoxy between IHS and core instead of thermal paste, this gives much better heat transfer. Trying to remove an epoxied IHS will destroy the processor, even though some claim there are tricks such as placing it in a freezer overnite. The reason Intel did this was for better thermal transfer, it's not as many thought, to dissaude people from removing the IHS. Out of a million CPU's sold maybe a hundred people might even try removing the IHS.
For Pentium's I've used the "string" method, where you find the corner of the IHS where Epoxy is absent, and use the string in a sawing motion slipping it underneath the IHS until the epoxy seal is gone. DO NOT FORCE THE IHS OFF! TWISTING it FROm SIDE to SIDE GENTLY is the safest method.
Thermal Paste application is also very important to temps. DO NOT SPREAD. Since the IHS is approximately 70% larger then the core beneath it, the heat is concentrated in the center. Yes the IHS is intended to "spread" heat by increasing surface area for that heat to dissipate over, just as your heatsnk is designed to spread heat. This is why the best heatsinks are behemoths weighing over 630-grams. More mass = more surface area, more mass = more area a fan can cool, drawing the heat into it.
PLACE a grain sized dollup of AS5 or Ceramique (or whatever you use) in the center of the IHS. Even if you've removed the IHS, place a smaller dollup of TIM on the center of the core. DO NOT SPREAD, simply mount the heatsink, and allow the natural pressure to spread the paste. WHY you ask? Because the heat transfer primarily occurs at the center of the IHS, and with a decent heatsink this heat will be rapidly dissipated. Spreading the paste over the IHS can actually defeat the purpose, as it it's akin to throwing a blanket over the surface. This is where the Convex IHS theory becomes doubly important. If (and many are) your IHS has a slight convex, then only that area will be making contact with the heatsink base, spreading paste over the entire surface of the IHS will be like "insulating" (throwing a blanket over) it holding heat in that would normally dissipate into the environment.
PS: by the way, I'm not trying to come off as a "know it all." I'm a very humble person, and I certainly don't know everything. These are things that others taught me, an that I learned form research. I wouldn't share anything with you I haven't done myself, and much of what I've learned I learned in long conversations with Nevin House of Arctic Silver. I've been lucky to call him a friend. I'm totally open to learning, and I still consider myself to be a noob in many areas.
but really the BIOS probably says 60-65C...