Flashguards are required by the Brigade of the Amereican Revolution, Continental Line and the British Brigade. Any AWI or F&I event will require them on muskets, but in some cases rifles not fired on line are exempted.
While they are indended to redirect the gases upward, they do tend to send some of it downward, thus fouling the lower part of your lock, the stock, sling and trigger guard. Not the end of the world, but as the others said, if you're not going to be firing it on line with other shooters, I'd leave it off.