Mildly buckled will be ok this will be just like fire forming the case again.
As mentioned annealing softens the brass. Brass work hardens when constant shooting and resizing the brass gets harder and harder and if it harden enough this where split case necks and case head seperation can happen. ONLY anneal the case neck and shoulder area if you go any further down the case you may soften the brass to much and when firing you may have a case rupture. Not nice.
To reduce the chance of case head seperation by constant FLS only neck size and when getting harder to chamber then do a FLS. Or even better as Apollo has said get a body die just to bump the shoulder back a thou or so to make chambering easier again. But I'm not sure if you can get a body die for the 7.62x54R. You can always get one made by CH4D,
http://www.ch4d.com might cost a bit though.
Now when seating the bullets and you are causing the case to buckle in the shoulder area I would say you haven't deburred the case neck enough. No bullet seating should be that hard that cause's the issue you are having.
Bill mentioned only the neck but if annealing at the right temperature at the neck the shoulder will get annealed slightly due to heat conducting down the case. But if the neck is deburred properly the shoulder shouldn't have any issue supporting the bullet seating process. As mentioned annealing must be done at the right temperature.
I have annealed plety of 303's to be necked down to 6mm and have no issue's for my next project rifle.