It's up to the organizers of the event. The only time I've ever been upset about a regiment firing while charging is when the rules clearly state that it is not allowed. If Firing on the charge is allowed then I don't mind, even if it can be annoying. Having commanded line and skirmisher units before I will say that it can affect your regiment's performance in melee combat if you let them hang back and shoot or if they try to reload, or take too long to shoot, etc. It can turn a well organized and swift charge into a spread out mess. I've had linebattles where an enemy regiment fired on every charge but it didn't matter because of 3 factors: 1) They sucked at shooting 2) They were too spread out to help each other in melee, and 3) Guys who took too long to shoot were left vulnerable and bayoneted to death.
My verdict:
If it isn't against that event's rules, then fire while charging - at your own risk.