One of the many theories behind not loading a magazine to capacity has to do with doing a combat reload whereby you remove a partially loaded mag and replace it with a fresh one. If there is a round in the chamber and obviously the bolt is forward, it requires a lot more pressure to seat the new magazine. Removing a couple of rounds helps it to lock in a little easier. "Easier" is of course a relative term depending on the gun used, shooter using it, training level, etc. Take your AK and try it both ways and see what you think. If it doesn't bother you, load them all the way.
Out of habit I load my M-4 mags with 28 rounds. I have done it sine I was a young PFC so it is more habit than anything else. My 20 rd M1 mags I load with 20 rounds because I don't want to give up the extra ammo.