That's an interesting question. I depends on the kind of modulation you use, and how precise you need it to be. For instance, if you modulate with a k-rate sine wave, and use conv/interp, you are going to get a "reasonable" version of a sine wave. But if you need a sawtooth with a vertical edge, you can't use lfo/saw cheap: the conv/interp will transform the vertical edge to a line, going from 64 to -64 in 16 samples. In that case, you would use an osc/phasor.