Wecome to FT, PRedLSU! There are lots of different viewpoints here on that. But everyone here would agree that you first need to understand what sort of seat is on offer for a flight you're considering. The starting point is the plane type, or equipment in local parlance.

Once you know your airline and plane type, head over to seatguru.com to see what the premium class(es) are like. The might be barely better than coach. Or they might be flat bed fantastic.

From there you need to make a judgment call about how valuable the premium cabin is to you for a given flight. Obviously, the shorter the flight, the less important it is to get out of coach. Time of day can be important as well. If I know that I'll be dead tired and in need of sleep, the upgrade is much more valuable to me than if I know that I'll be awake throughout the flight.