When it comes to breaking-in a new pipe I'm very old-school. Since I prefer larger pipes, I fill it about 1/4 full for the first 5 or 6 bowls, then 1/2 for 5 or 6 more ending with 3/4 full for 5 or 6 bowls. I know that many of the current "experts" say that this isn't necessary. They could be right, but I know that this procedure has always worked well for me.
Building a good cake is crucial to ending up with a pipe that smokes well. As you smoke a pipe, the moisture in the tobacco is converted to steam, which carries with it a lot of the tars that are in the tobacco. Some of this steam/tar mixture escapes from the top of the bowl, this is what causes the build up on the rim, Unless you're using a torch lighter (DON'T use a torch lighter on a pipe. EVER!) that is not "charring", it's accumulated tar and will clean off easily if you don't let it build up. The majority of the steam/tar is drawn through the pipe and into your mouth. When this steam hits the heel of the bowls it slows down as it is forced to change direction into the shank. This allows some of the steam to condense, which causes water and tar to accumulate there.
It's critical to the long-term "health" of your pipe to build a protective cake in the heel area. If you don't, the wood will absorb that moisture and tar. Be honest, if you start out a new pipe with a full bowl are you ALWAYS going to smoke it to the bottom? Probably not. A new pipe burns hot and even the best of them have a somewhat unpleasant taste until they begin to season. If I just spent $100 or $200 for a pipe, I want it to give me the very best smoke that's possible. For me, it's worth using a little patience to do it in a way that I know will help ensure that result. It may not be necessary, but in 44 years of pipe smoking I've broken in several hundred pipes so I'll continue to do what I can to protect the investment I have in my briar.