I used to work for a company that did this. I worked summers while I was in college and have sealed many driveways. My advice would be to have it done professionally. We would drive by the DIY jobs and you could definately see the difference. Doing it properly takes more prepwork than most people are willing to do, ie. trimming back all the edge grass, cleaning the asphalt, priming any oil stains and filling cracks with special filler.
We would recommend sealing every 2-3 years depending on the amount of wear and tear on the asphalt. If you do it too often it will build up and flake off.
Not sealing to allow the asphalt to 'breath' only applies to freshly paved asphalt. After it cures it's not going to breath any more and you want to seal it to prevent moisture to get into it, freeze and then crack it.