I wouldn't lose any sleep over this. one of the most charming things about Italian cookery is trying to get your head around all the funny little rules and rituals Italian cooks love to wax lyrical about. I can't claim to be the world's most authentic pasta chef but I cook it a lot and I nearly always add sauce to pasta! There is a simple, practical reason for this. You usually need a big pan or pot to cook the pasta and a small pan or pot to cook the sauce. I return the pasta to the pasta pan and use it like a large mixing bowl. One important thing to remember if you are stirring sauces through pasta is not to drain it too thoroughly. A couple of shakes in the colander is all you need. Pasta coheres more effectively with any sauce, from pesto to bolognese, if it is slightly damp rather than dry and sticky. There you go, another quirky rule...can't help it!