+1 on BB as to the reason to do it. I have mounted a number of scopes and have never lapped any. That being said, some could have used it a little but I examine each sitting in the lower part of the ring to make sure the the rings are true. No need to kink a scope. Just need to be careful. Different kinds of rings may not allow the technique I use.
I lap all of mine....it doesn't take long and it can only help. If you do lap your rings and material is removed, it generally will get you more surface to surface contact between your rings and scope. Some lighter rifles it may not matter, but the bigger bores I think I will take all I can get to keep the scope in its place.
Right now I use Talley 2 pc. I use the alignment bars to see if they are squared so the scope sits in there level. The last 5 scopes I have mounted required just a little bit of lapping. After that they were perfect.
I used to lap rings, thinking that it can only help. That is kind of true. You do usually get more contact on the scope tube. The problem is if you remove the rings, and remount them (like if you swap scopes between rifles - which I often do) then the lapping will likely no longer be true, because the rings were lapped to the previous bases. Even if you replace the rings on the SAME bases, the rings will likely not take the exact same purchase on the bases. Not a huge deal, of course, as you can just re-lap the rings after mounting them back to the bases.
If I am mounting to two piece bases, I prefer to bed the rings to the scope tube, giving nearly perfect contact, even better than lapped rings.
Still, bedded picatinny rails and unlapped rings are my preferred method. Then I can switch scopes easily between any of my rifles, without worrying about anything.
A quality set of rings like Seekins require no lapping. As they are machined as a single piece and only then split into a matched pair. Mounting to a picatinny rail like Matt mentioned is how we run our rifles as well. Never lapped a single pair...
I just got a new book on rifles by John Barsness. He goes through several aspects of rifle shooting and reloading. One of them is lapping scope rings (he doesn't suggest doing it) and he essentially said the same thing as some of the posts above: get everything else in a straight line and it is not needed. And even if it is needed, there are other ways to solve it that will work better in the end.
For those of ya that bed which ever kind of base, what do you use for your bedding compound. I have talked with guys that use JB Weld, bondo, and regular bedding compound. I know there are a million ways to skin a cat, but which would be the less Reddneck, or correct compound to use?
I believe that the one piece bases help with scope ring alignment. When you look at the grand sceme of things, if you have scope torque it may not look as if it is enough to effect a shot. Think about the amount of pressure that small torque is under during the shot. If you are concerned enough to think about lapping your rings you should do it. I started worrying about scope ring alignment but it doesn't end there. If you worry about scope torque affecting 1 shot out of 10 then you should worry about the other "Flyer Causing" defects in a factory rifle. Bolt lug impact, recoil lug impact, bolt face squareness, and action bedding. All of these items are easy to repair without expensive machinery. If your gun is making 100% contact where it should be and all is square and true. The gun can shoot the same way every time due to what I call "Trueness". Then it is up to you to make the shots you need to make. If you true your action you are a barrel installation away from a custom rifle. I will warn you if you attempt these truing repairs. Keep in mind headspace is the most important thing to keep track of. The guages are worth the 60 dollars from Midway USA.