Just about to start a RTW - I had planned to only use 15 segments and held back from using the 16th in case I had an 'involuntary change' forced on me (say having one of my direct flights cancelled and having to take an indirect route)

I would like to use the 16th segment but am I taking too much of a risk ? Should I hold it back in case or will it be OW's problem if I get a flight cancelled and have to exceed the 16 limit ?
Thanks