It depends... How much of a reduction in gearing do you need for a certain incline? How much momentum do you have going into the climb? Do you like to spin or to mash?

For a steep or long hill that's going to require some work, I might stay in the harder gear as long as possible, and then shift down the front first . For a short hill where I've got a lot of momentum, I might only shift once or twice in the rear and power over the top.

Unfortunately, there's no magic formula. Listen to your legs and lungs....let them tell you how much easier they need it to be.

The scientist in me likes to experiment with things like this, and understand how they work for me. I think I would find a practice hill nearby, one that isn't crazy hard or very easy. I'd practice riding that hill over and over, in all sorts of gear combinations, and see what happens when shifting just the front, just the rear, combinations of both, and in different orders.

Have fun, and good luck!