My dads both needed bedding in the rear. If you can hold the rail down on the front of the reciever and the rear has any gap, the rear at least needs bedded with only the front screws torqued. If the front is up when held down at the rear, the front at least needs bedding with the rear screws torqued.
If there is any cantilevering from the reciever not being parelell and or not being on the same plane, the screws will force the rail to bend and thus make the ring bores out of alignment unless lapped well, or bedded. If far enough off, lapping may not even correct it, and possibly even keep the rings from tightening on the tube completely. The other problem with lapping to fix "this" problem is, when and if you remove the rings from the base to swap the scope to another "like" rail on another rifle, you can bend or crease the scope tube when the rings are torqued to the base because they're lapped to fit a bent base. This of course will happen if the two bases are not bent exactly the same. Cure? Make sure they are both true and bedded properly. A magnetic base and dial indicator will tell you if the base is being sqeezed down to make contact the reciever, bowing up or down in the middle etc.
Jared Go to snipers hide and do a search on Kens bases there was a problem with the screws, that he corrected. But the bases have a slot for the bedding to go in and the screws are unsupported, some have sheared off that were not bedded. Iam kinda fuzzy on this so check it out, for yourself. ebb
I have two of Kens bases a 30MOA and a 20MOA both bases fit both of my guns perfect. No bedding needed, they have not moved since installed, I did use blue loctite on the screws though. Kens bases are well made and well worth the money he gets for them.