fine tuning your cadence is a peronsal matter

spinning too fast or too slow is inefficient and will over-work your cardiovascular system or your localized muscles respectively (i.e. too fast and your lungs will burst, too slow and your legs will explode)

the key is to listen to your body (and use a heart rate monitor or power meter or both) and find a good balance of gear size and cadence. when you ride, watch your cadence along with how you feel. if your legs are pumping out, but your lungs and heart have energy to spare, pickup the rpms, and vice versa.

if you are searching for extra speed on your rides, it is easier to teach your body to ride faster at a higher cadence than to ride at a lower one. your cardiovascular system recoveres and rebuilds more quickly than your muscle fibers. if you overstress broken-down muscles you run a higher risk of overtraining and knee injury (as knee injuries tend to occur when the muscle is so fatigued that the stress it would normally absorb goes to the joint). that is not to say you shouldnt work on building more muscle and your capacity to push bigger gears, that too is important, just takes longer to achieve and must be done with a little extra care to avoid injury.