I have had the same issue for years. There is no perfect solution.
If you leave it at "no change" so Movies are in the format intended, older tv shows in the 4:3 format have big side black bars.
If you set it to "stretch", the 4:3 tv looks fine (but stretched, but I am used to it.). A movie not full frame, like a 2.35:1 will be stretched vertically and all folks will be really tall. I don't see any trigger to tell MC to stretch or not, just on or off.
So we leave it at stretch (as we watch more tv than movies), and if we start a movie we use the arrow keys to chose "aspect|no change".
Inconvenient, but we have done it so long I consider that "how it works" and not a bug
There is a tag for "dimensions", perhaps a rule could be made so anything that doesn't have a TV fitting Ratio, gets stretched if needed, or not.
I have been playing with zones, maybe a Zone for one and the other?
I will play with it myself, I make progress I will report back.
Good Luck
Don