Can you give some examples of what the theater has asked of you?
Could it be that they’re just communicating with you about tasks that need to be done by first rehearsal, and they don’t really expect you to do them immediately? If the tasks they’re giving you have deadlines that fall before your official employment starts, I think you could negotiate compensation for doing them. And if they’re asking you for enough things that you find yourself counting the hours your putting into the project ahead of time, that might be a sign right there that it’s too much.
I've worked on SPT shows where I’ve attended production meetings well before my official employment starts (without compensation, though I suppose I could have argued for it). I’ve also met or talked with directors ahead of time to get their ideas about scheduling, and called actors to find out about their conflicts before Prep Week. Often I’ll start paperwork, such as rehearsal report templates, sign-in sheets, and a preliminary prop list, before Prep Week. But aside from the production meetings, which the theater has asked me to attend, the other things I do are for my own benefit, my “pre-prep.” It helps me to have some extra time to wrap my head around the play, and starting a few things early makes it less likely I’ll get overwhelmed during Prep Week itself. If the situation allows it, I also like to work 5 days rather than 6 during Prep, so I figure my time balances out.