Recently a new SM asked me to provide him with some good rules for rehearsal-what he should/shouldn't say, etc. After giving that to him, I thought some folks might find it useful. (Just so you know, I'm a professional SM currently pursuing my MFA so I've been doing this for awhile. 
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1. Most directors want stage managers to be seen and not heard. Unless you're asked, you should never offer your advice or an opinion. You never want to tell a director how to do their job-you can offer the problem, but don't offer the solution unless you're asked.
I disagree with this point. Whilst I NEVER offer creative suggestions unless asked, if I see a problem, ie something just will not work (ie an action requires defying the laws of gravity etc) I will point it out, as well as possible solutions - taking care to never seem like I am influencing the way the show actually looks. I am also strongly of the opinion that we are more than just the directors Personal Assitant, and that whilst the creative control is in the directors hands, I will take charge on the organisational front - which means that I will call out for order, I will make announcements at the start and end of rehearsal etc and inject comments on things the director may not know which could prove valuable to everyone.
2. Try to anticipate the director's needs and requests-it's super helpful if you can be one step ahead. I always love it when a director asks me to take a note for a rehearsal report and I've already noted it.
3. Never answer a question with "I don't know". If you honestly don't know the answer, say "I don't have that information right now, but I will get you an answer as soon as possible."
These two fall together - I could not agree more, although your offered phrase is a bit long. I usally say "I don't know off the top of my head. I'll get it for you later" - and write it down as I say it.
4. Try not to fidget immensely or create lots of excess noise during rehearsal. Un pack everything you'll need, unwrap any snacks, etc. Lots of directors are noise sensitive.
Good point. It is common courtasy - you expect the cast to pay attention to you when you speak, the least you can do is not interup them.
5. Make sure you know where all the lights are as well as temperature controls-as much as possible you want your rehearsal space to be well lit and comfortable. If you don't have control over the temperature, find out who does so you know who to talk to.
In addition - during extream weather conditions (ie quite hot or cold), make sure you (or someone else) can get there an hour or two early to put on the heater/AC. Also, rehearsal rooms can quite often become musty in smaller venues - it is worth opening the windows a few hours before rehearsals too.
6. NEVER bash a director in front of your cast, or a cast member to an actor. If it's happening and you're present, simply don't take part of the conversation. If someone asks your opinion, change the subject or just say you'd prefer not to comment.
I would go one step further, and stop the conversations. That sort of behaviour should not be tollerated - especially in professional settings. I mean these things have a way of getting back to the person they are about.
The saying "Sticks and stones may break my bones, but words shall never hurt me" could not be more wrong. About my 6th show, a teen theatre (all cast and crew < 18yo), there was an actor who was bi-sexual, and was probably the worlds largest flirt (although I now know, from a reliable source, that he was a virgin). Well, one day I was in the rehearsal room, doing some pre-rehearsal paperwork, when a few of the cast came in and were bagging this actor. I did not say anything, partly because I was not paying close attention, and partly because I was still not an overly confident SM and did not want to desert my cast. The actor came in barely a minute later and heard almost the entire "bagging" session. I saw no ill effects - I did appologise to him for not stopping them, but he waved it off. I thought the matter closed, no harm done. 3 months later he had slit his wrists. Words are powerfull, and a bit like water. Their effects may not be seen instantly, but over time, and with lots of them, they wear people away.
This list could go on forever, but these are some of the immediate things I thought of. I hope this is helpful to others!
