There are no rules. It's all about personal preference, fitness and terrain. Whatever let's you ride comfortably and meets your needs is the right gearing for you. I started with a triple crank when I started riding 2 1/2 years back. After a while I would rarely ever need the 30T, so went compact double. My 34T has been nice because I get the double style shifting, but it can get me up stupid steep grades round here, even in the 20-30% range. which I couldn't do with a 39T chainring. 11-12% is a good climb, but not crazy steep.