rule of thumb is to carry a spare prop when you do not see land ie offshore fishing. I dont however we used to carry one on our bluewater trip with a yamaha wrench. I say used to because once you go with a duel engine, then you can limp in with an engine + hail sea tow.

I dont feel a need to when fishing local or even nearshore with my boat as long as I have a trolling motor, AND a radio.