Where I have ridden on some difficult hills, my bikes have always been able to downshift under heavy load. That said, it's bad practice. However, there are desperate times when you get caught off guard or you find out about half way up the mountain that you're more tired than you thought. Downshifting one cog at a time will not blow anything up. But if you just slam the shifter and expect the chain to just jump up 2 or 3 cogs you will probably break something.
-F
OP lives in my area. He's not dealing with climbs that long. Terrain here tends to be pretty rolling. Climbs tend to be short, but somewhat steep. If you're strong, yeah, on many of them, you can hammer on the leadup, and then really mash to finish the climb. Some of them don't quite work that way. The lead up to the climb might be a slow segment of trail, or the climb might be just a shade too long to mash the whole way up.
The question becomes not whether a downshift is going to be possible, but deciding when to do it and making it happen smoothly.
Some local places (that the OP is not riding yet, AFAIK) have climbs that are steep enough that a lot of riders have to get off and walk. You'll start off with some nice momentum from a downhill and the trail immediately kicks upward. Your mo' carries you part of the way, but you've gotta start pedaling at some point. AND you have to dump a lot of gears, because the grade kicks up in excess of 30%. Dump too late, and you get caught at too low of a cadence, the shift doesn't happen, and if you try to force it, you break $hit. Dump too early and you're spinning out. I usually wind up dropping gears slowly by soft-pedaling while my earlier momentum carries me up the beginning of the hill. My final gear choice depends on the hill I'm tackling. If it's a really steep one where my momentum won't carry me anywhere close to the top, I'll go clear down to granny. If my momentum carries me closer to the top, I might not drop many and I'll power up it. It all depends.
In short, there's some trial-and-error involved to learn what the bike's capabilities offer you, and to learn what YOUR capabilities are, and then you have to figure out how to interpret the terrain and conditions into the right shift. You will mess up. I agree that it's better on the bike to mash than to shift under load. But you're going to have to practice softening up on the pedals just enough to allow the bike to shift. If you don't quite pull that off, then it's far better to get off and walk than it is to try to force the issue and break something.
