Be definitive, know what you’re getting yourself into, control scope by deciding yes or no. Maybe is a scope expander, a deadline wrecker, and an appeaser that ends up being a displeaser in the end.
[My wife and I use a binary system about all household spending (which for us is pretty broad category of purchases). We decided early on that only one no “wins”. The revisit period is indeterminate, but any agenda item can be brought up again if circumstances change. There are certain revisitation exceptions like fabric patterns. It’s easy to get talked into something, but you’re never really going to like it. It’s not as formal as it sounds and it works for us. YMMV]