When should or shouldn't you toss pasta with sauce?
It really depends on what your goal is; that is, what you are doing with the pasta.  If you are making most authentic Italian pasta dishes, the slightly under cooked pasta is transferred to the pan in which the condiment was prepared.  It is then tossed with some water from the pot that that the pasta was cooked in, along with some butter or olive oil (depending on the region of the dish).  This finishes the cooking, emulsifies the sauce, and mixes in the the condiment.  The final dish is not about pasta or the condiment in isolation, but rather the combination.  Just like a salad is not about the lettuce, per se, but the combination of lettuce and dressing.  It is true that pasta will potentially absorb the sauce and get mushy if stored in fridge.  This is less of a problem if sauced appropriately, but still, it is possible.

First choice, prepare only enough pasta for the meal.  Depending on the type of pasta, you are only looking at a maximum of 10 - 12 minutes of cooking and often less. So, it's not really about convenience. If you find you have cooked more than you need, you could always chill and reserve non-sauced pasta, then reheat it in condiment at another time.  But, in my opinion, this would be inferior to freshly cooked, and take almost as much time.

If your goal is a cold pasta salad, by all means, cook, chill, then mix with dressing whenever you need it.