What do you consider a "real" start of DSP?
Toggle DSP button ON, aka: ";pd dsp 1"
Actually, I'm rather asking myself whether it's necessary that saving triggers a rebuilt of the DSP graph...
I believe connections need to be re-asserted/ordered so as to re-draw the graph. Same thing happens with non-tilde objects on canvas.
To add to your question: if no tilde object was touched/moved (or even exists on a patch) why would the dsp graph be rebuilt on save?