Wellll...not just mod sources, tho. Knowing the oscillator (or better, "audio generator") compliment does go quite a bit of the way toward determining your modulation needs. Filters, VCA...those are sort of givens. But if you do a build with a pair of complex oscillators, these need different modulation schemes than would, say, a wavetable oscillator plus a very basic VCO for sound fattening. Or two (or three, or more) plain-jane VCOs in general. It's actually one of the stumbling blocks...when picking the audio gens, you do have to have part of your thought processes working a few steps ahead, and that's not always the easiest bit of multitasking.