not a rule, no FCC regulation says you have to use phonetics. but there's a reason everyone does it -- letters sound alike on a bad signal. B and D and E and V all start blurring together when theres QRM or youre on a marginal path. NATO phonetics specifically were designed so that each word sounds as different as possible from every other word, so even if half the audio gets chopped up the other person can still figure out what letter you meant. the situation you described is exactly when you should reach for them. if net control is asking you to repeat, that's the cue to slow down and go phonetic. doesnt matter if it feels weird at first, after a few months it just becomes automatic. some guys also mix in non-standard phonetics which can cause confusion, like using "America" for A instead of Alpha, so sticking with NATO is the polite thing for interoperability.