When conducting a rehearsal, do you use 'you' or 'we'?

That's known as a "rhetorical we."

I don't know what it is, but my work as a teacher has caused me to use the rhetorical we.

I would say, "Remember what we talked about yesterday?"

when really, I was the one who did all the talking.

As in your examples, I would also say "we" when I mean "you."

When I was a student at Interlochen, there was a recurring joke about the rhetorical we.

Students greeted each other by saying, "How are we today?"

"Just fine, how are we today?"

"Just fine, thank us."

"We all need to play forte at G." It wouldn't make sense for the conductor to say something like that because he can't play anything forte. Rather, something like "Make sure G is forte" or "G should be forte" would be better. Even "We need a good forte at G" would work, because everyone (players and conductor, and audience) will need the forte.

With anything else, the conductor could give directions using whichever subject (you, we) he chooses. I hate to break it to you, but the conductor IS above the rest of the orchestra. He's the one in charge; he's the one making the decisions. What is "patronizing or reducing the conductor's authority" is when the conductor doesn't take the leadership position, avoids making command decisions such as clearly instructing how he wants something to sound, or always asks if it's okay to do something again. It's no fun playing under someone like this.

Inclusive "Let's go back to the beginning and go straight through" or directive "Take it back to the beginning for a run-through," any instruction should be given with respect toward the players. That is how the conductor will earn the respect of the players, who will want to take the directives of the conductor. Conductors who do not respect their players as clearly demonstrated through their comments are no fun to play under either.

I can't add much to tri.du's answer; but I remember Leopold Stokowski rehearsing the National Youth Orchestra of Great Britain. In a piece by Tchaikovsky (I can't remember which) one of the trumpets split a note. Stoky addressed the orchestra, saying, "From letter F, please; and this time, the trumpets will play *correctly*". (No "you", "we", "us", etc.)

Well when you want a violin as an example do something (e.g play more forte) use 'you'. But when you want all player to play more forte, use 'we'.

It is considered polite to use "we"