It makes sense on some flights. On 752s I don't worry too much. If a 752 is subbed to a smaller plane (319/320), there will be such a significant reduction in F-seats that you'll probably get downgraded if you're not paid-F. There are some common gotcha's though on similar type aircraft I tend to be cognizant of. For example E+ vs E- depending on the model of the same class airplane or ending up in seats that could become "crew rest".