三人行，必有我师(焉)

since I don't think I've noticed any instances where using the wrong one would change what character you'd written

Yeah, as far as I can think of the difference only really comes in relevant* when they're used as individual characters… but unless you're reading 论语 on a daily basis (子曰 blah de dah) it doesn't exactly come up often either.

* Unless you have the kind of teacher who marks you down for stroke inaccuracies (so, all Chinese teachers ever??) in which case it still matters. On which note I'd say 曰 is the "correct" radical here for the Mainland Edition™ of the character, since I recall being taught not to close(?) the horizontal stroke inside…