Front rims tend to stay cleaner than back rims, and dirt is abrasive so (as well as being good technique) a preference for the front brake will extend the life of your pads.
On downhills it's a bit different - heat is an issue not just for pad life but for safety. Hills round here tend towards short and sharp, so I use the back brake (lightly) for not going faster and keep the front for stopping. On longer descents I rest the back brake by giving the front a turn.
There are good reasons to avoid going fast-slow-fast-slow:
- following traffic (whether cars or bikes) won't expect sudden slowing on a clear road and you (presumably) don't have a brake light
- as your stopping distance increases a lot with speed you don't have to go much faster for hazards (like side streets because many drivers don't understand bikes that go quick) to get more significant.
Even good brake pads (which last longer than cheap ones) aren't expensive, and it's easy to change them yourself (front or rear in a session, then test, at least until you're experienced) so pad wear shouldn't be your top consideration - riding for the conditions should be.