I disagree with most of the posters.
A trumpet is designed to be played with normal equipment, i.e. mouthpiece, lips and hands. Every change to that setup alters the way the horn plays. It is not going to respond the same with a valve guard as without. The valve guard deadens the whole valve cluster, which is essentially the "heart" of the instrument.
A valve guard soaks up the acid perspiration and then, by releasing it slowly, allows it to attack the finish 24/7. If you wipe the horn down, the chemicals are no longer present to attack the finish.
Many valve guards are quite stiff and will rub away at the finish wearing it off.
Using a valve guard on a well designed trumpet is akin to buying a superbly sculpted Porsche or Ferrari and covering over all the front bodywork just in case it gets scratched.