What lookingfornow said. ^^^^^^ There's a zillion things that can happen in any given encounter. It's virtually impossible to discuss all possibilities beforehand and establish rules for all of them, much less remember all of them.
For my wife and I, we didn't have a huge rule set when we started. What rule set we did have has since dwindled. It's much easier to remember when most anything goes.
Communication about limits is important. I imagine what the "biggies" are varies from couple to couple. But, where the guys get to cum is probably on the list for most people.
We make it clear to guys we play with that my wife, though she LOVES giving oral, does not like to have guys cum in her mouth. She can do it, but it's not her favorite thing, so she saves that for me.