I don't count bills that I have set up on autopay (vehicle payments, mortgage, utilities, etc.). I also don't count gas, as I do very little driving other than to and from work or my association meetings, both of which are required. Most of my errands can be done along the route on the way home, not taking me more than a few miles out of the way. I also don't count my association meeting expenses, which are reimbursed.
Although I count things like groceries, errands (postage, pet supplies, haircuts, etc.), I'm not super concerned with them. After all, if I'm out of kitty litter, the money needs to be spent!
My primary purpose in participating in the no spend days is to restrict my spending on lunch/takeout (far and away my worst bad habit for frittering money), and other truly frivolous things like vending machines or magazines. That's what I really want to curb.