Good question, Barb. It's largely a matter of experience, I think, because after awhile you'll get a good feel for what size of hills you can manage in your middle ring completely and which are so long or steep you'll definitely need "granny's" assistance! If you see a monster hill approaching, you'll want to go to your inner ring before you max out in back, that's all. But many (most?) hills you may well be able to climb without ever having to shift to your granny ring. Once I make the decision not to go to my easier ring, I feel free to use all the cogs in back, knowing that I've now committed to not downshifting since I'll probably drop my chain if I do!

Since you live in a mostly flat area, you may not get a chance to practice and refine this skill, which could make for a bit of a challenge should you travel to a hillier area for an event or tour. But it's not a difficult thing to master once you get the hang of it.

As for shifting into a more difficult gear, on a descent you'll want to go to your outer (52-t) ring a couple cogs before you max out in the back, and that's a good practice anyway since normally you'll start hearing your chain grinding as you get to the smaller cogs in the back in the middle ring in the front. The other alternative is just to stay in the middle ring til you run out of gears in back, then coast!

So yes, there are definitely some gears that you will rarely use. You want to avoid excessive chain stretch (and noise) that results when you're in the granny and middle ring in the front and smallest cogs in back; and vice versa, avoid the larger cogs in back with the large ring in front. Effectively, a 27 speed bike is really more like a 24 or even 21-speed if you use only the gears that don't make noise and don't result in excessive chain stretch.

Does this help? (Sorry it took awhile; I've been away from the boards for a few days.)

Emily