Personally, I wouldn't cook spaghetti sauce, or any other sauce, in a pressure cooker. The original intent of a pressure cooker is to decrease cooking time and make tender tough cuts of meat. High altitude cooking (braising, for instance), almost demands the use of a pressure cooker for select meats (usually roasts) and stews.
In my experience, homemade spaghetti sauce benefits from simple long time simmering on a very low heat with occasional stirring. I don't think using a pressure cooker would gain you anything flavor-wise, but that's just my opinion.