I worked for an asphalt company, that refused to do sealing, and my boss said that the sealer is just a gimmick to make money. It is a repeat application that is completely unnessary. First the company gets you to seal the driveway to protect it, then they get the follow up every few years. The alligator cracking that most people see is do to the sealer cracking, not the driveway. So for asthetics, people are resealing the sealer, under the impression they are doing something for the driveway. Look at a roadway, much more abuse and no sealer applied.
If a driveway is installed correctly (enough blend, remove roots) it shouldn't be necessary to fill cracks. The most important part of driveway longevity is to get the state mix of asphalt not the typical commercial mix that most companies use on driveways. It may not be as pretty and smooth when it's done, but it is much better quality.
Some people just have to have that nice black look, so I would do it for them, but never recommend it. Whenever I did a resealing job, we used the pails of stuff from Home Depot not the hot tar, we just poured some out on the driveway and then squeeged it around to fill in. Make sure you use a leaf blower to remove all the dirt and dust, pick out all the weeds, and some products (latex sealer) requires you to wet the area first.