So, to summarize [and correct me if I'm wrong, because I'm learning, here, too ;)]:
Outside of the equipment enclosure [i.e. *down the MIDI cable*] it really doesn't matter if there is 3.3V or 5V in the box. All that matters is, there is enough current to drive the receiver's Opto Isolator -- and that is a nominal 5ma.
In other words, whether it's 5V or 3.3V is obfuscated by the resistor selection, which mandates the common [i.e. *backward compatible*] 5ma signal. So, a 5V derived signal, can drive a 3.3V receiver, and vice versa.
Am I right? Or am I right?!?