You want as little play as you can get. It should still be reliable if it is loose, but accuracy will suffer.

On my garands, I slightly peen the barrel to hold the Block tight (gas block not pulled very often).  

On NM rigs, the barrels are peened, and the block is red loctited on to hold them rock solid.
These rigs are only take apart when it's time for re-build.

If your shooting corrosive ammo, and need to pull the gas block, then live with the slight wobble.  Repeative peening of the barrel is just going to wear it out/make it worse really fast.