M16/A1 Bolt release
Rapping the mag against the helmet or other hard surface to ensure the rounds are fully rearward in the mag is an old trick, and a common one. Remember though, it only works if you hold the mag correctly when you do it. Rap the front of the mag and you can defeat the purpose.
As to using the bolt release to load the rifle after inserting a new loaded mag;
Yes, certainly. It is the fastest way, provided the bolt is already locked open. Insert mag, hit bolt release, hit forward assist, then rock & roll.
Using the charging handle involves adding a seperate motion (pulling back the charging handle, and it needs to be pulled back ALL the way, otherwise the bolt release will not disengage) and also creates the possibility that the shooter may "ride" the handle instead of letting it fly forward freely, which can result in a failure to fully load. All in all it is simpler and faster to just hit the bolt release. On the other hand, anyone who had ever messed with the M16A1 enough has also learned that a sharp rap on the butt will jar the bolt release enough to let the bolt shut.