Depends on the situation...
This following text is based off of Cod4 by the way..
In most cases, you have to realize that usually you will find yourself in a hot zone... Which is when the action is high and both teams are hitting each other very hard in a given area of the map to try to push the other back...
In times like this, I only reload when I'm forced too... Meaning I emptied my main AND my pistol, and found any upcoming enemies out of reach for the knife... I've found myself ready to launch a chopper with this method the moment I either get away, die, or we succeeded in the push and the enemies are making their way back for another round, by that point I'm fully reloaded and ready to greet them in the challenge... Once I'm fully out of bullets and don't see any knife victims, I'm out of there! I retreat for a moment, reload everything and continue the push.
However if we fail in the push, I'll immediately retreat to an area we have dominated to keep my k/d low, reload when I feel it's safe, and take my hand at flanking the opponent to try to help the team regain lost ground. Either that or if I'm on LMG I'll find a good choke point and defend what we still have while the others make their push.
If by chance I find a few enemies in our zone trying to flank or whatever... I usually feel it's safe to reload the moment I send them back to their own side.
This method and a few other tricks I've picked up along the way have kept my k/d ratio in the area that others are quick to call BS!!!
[small]No this isn't bragging, it's an example to help others in their playing of the game. I could post some end round results once I head down to the house this weekend if anyone is truly interest..[/small]