I think the answers you've gotten are good, but more geared for someone you are paying, not another parent. I would set simple rules, rather than have long discussions.
The main thing is to think about what you'd expect someone to ask you if you were there, and things where you've seen other parents do something that you wouldn't. For example, my daughter has a friend whose mom used to be uber concerned about feeding him only healthy food. She's now swung the other way. Earlier, I gave my daughter she wouldn't give her son. Now she gives him cookies and potato chips fairly regularly, while for us those are a special treat (cookies) or just not offered (potato chips). If I left my daughter in her care, I would expect that she not feed the kids cookies or potato chips.
1) NO TV, movies or computer/computer games without prior consent from the "sittees" and duration and content must be discussed in detail.
(I once made the mistake of showing Wizard of Oz to a kid I was babysitting as a teen. I turned it off within the first 5 minutes because he was terrified.)
2) no sugary snacks, potato chips, cake, cookies, etc without express permission from the parents. If you have something planned let them know in advance (as in, I thought we'd all make brownies!). If it comes out of a box or a bag, you should probably ask first. In Seattle that would almost certainly include "I was planning to serve hot dogs & Mac & cheese for lunch."
3) On a similar note, if a meal will be provided, you should probably let the sittees know what you are planning.
4) no drinks other than plain milk or water. Juice, chocolate milk etc, should be discussed prior. Soda or other caffinated beverages are not OK without express consent.
5) on the other side of the coin, if your kid has non-standard restrictions or allergies, let the sitters know. For example, any fruit should be OK, unless your kid has an allergy to say, Kiwi.
6) If you are the sitter, you do not drink alcohol or ingest any other substance that might impair your judgement. You do not leave the kids for any reason, nor do you leave them in someone else's care. If you might have company, you must clear it with the parents first.
7) any activities outside the home must be pre-agreed. (We might walk down to the park at the end of the street should be sufficient).
8) bathroom/diapering issues should be discussed, as well has hand washing protocol. You insist on hand washing before and after meals and after bathroom. Do you also insist on a full hand wash after playing outside? Other times? Is using a wipe sufficient? How do you feel about Purell?