Mashing or not, granny gears or not depends on one's comfortable cadence.

My comfortable cadence is around 95 - 105 rpm, but I can keep up 115rpm for 15 minutes at a time.

I tend to select a gear which is in my comfortable cadence uphill or downhill. While going uphil, if I get tired and slow below my comfortable cadence I shift down. That's all there is to it.

Regarding mashing, it's simply style. Lance Armstrong will spin 100+rpm uphill in low gear while Jan Ulrich will spin around 60rpm in very tall gear going uphill. It's all matter of comfort.

On my commute, I sometimes runinto a guy built like a tree who will mash a very tall gear at around 2/3 of my cadence, but keep up and most of the time pass me (that's with me trying very hard to keep up).

Far as I'm concerned, I cannot mash big gears that's all there is to it.