My view is that the number of bars ahead you need to be ready depends to on the speed of the music. I generally aim to have my horn ready, with me looking at the conductor a few seconds before I make an entry, so that if the conductor decides to specifically cue me, he can make eye contact and be confident that I am following him.
As for putting the horn down at the end of a passage and start of a rest, this very much depends on circumstances. Normally, I keep the horn up for a second or two after I finish a passage, just to make sure that the horn is completely steady until I have thoroughly finished the last note of the passage. It is too easy to crack the last note of a passage because you are already getting set to put the horn down.
If there is only a few seconds rest until the the next passage, then the horn will stay up and ready, but I might take the mouthpiece off my lips for a moment. If the rest is longer, I will slowly and calmly put the horn down until it is next needed.
There are exceptions of course. For instance, the horn will go down very quickly in the following cases:
- If I have only a short rest in which to turn a page
- If I need to empty the horn and don't have much time
- At the end of a movement, until the conductor puts his baton down and relaxes
- At a general pause, until the conductor starts the next passage
If you are playing in a concert, you are providing a visual spectacle as well as an aural experience. As far as possible, your movements when not playing must not distract members of the audience from their enjoyment of the music and of the playing of your colleagues. So you avoid sudden movements where possible, and when there is a silence, a moment of stillness
in the music, you absolutely must not break that stillness visually or aurally by making any kind of movement at all.
Although this really only applies during concerts, it is good to get into the habit of doing it all the time in rehearsals as well so that it comes completely naturally to keep still where required on the concert platform.
Why avoid unnecessary movements when not playing? Primarily to avoid distracting the audience (who are after all paying to enjoy the performance). Doing the same in rehearsal is simply good practice - i.e. practicing doing it right. If as a result you reduce the causes of distraction of other players, so much the better.
I take the view that whether the players are being paid or not, the audience is paying to listen, and that they therefore deserve the best performance the players can put on, in all aspects. This might be thought of as a small detail, but it is a detail which is easy to get right, and good performances come from attention to a succession of small details.