I don't think I ever met a conductor who would openly disapprove of foot tapping unless it were loud or maybe someone sat on the very end directly in front of the audience and did it in a very distracting way. The alternative of musicians keeping poor time would be far worse.
it's considered very bad form, especially in orchestra's , _especially_ by the conductor because it means 1. you're not following them 2. it's very distracting for the audience/other members of the band 3. it's a 'beginner' tendency, shows lack of self-control.
I've played in a few orchestras in my life and never heard that opinion (other than the part I previously mentioned, when it becomes a distraction to the audience), but I suppose some conductors may feel differently.
Perhaps a different culture between string players and brass players? Or maybe just different parts of the world? Eastern US here. I play trumpet and from the earliest years was heavily encouraged by all band directors and conductors to tap my foot at all times. No conductor ever corrected the practice later or pointed it out as an error, but I often heard the advice repeated for everyone to do it. Maybe they viewed it as a necessary evil though, like in Mr Holland's opus when when he is beating the tempo into the guy's head with a mallet while he wears a football helmet.
