We use tags and it works great. It's easy to use, lives across milestones, and easily editable. We enter "buildnnn" where nnn is the build number.
Just remember that you will want to start with 001 because build1 will also bring up build 10, 11, etc. - Another good way to number the builds could be yyyymmdd.n - example: 20100206.1 (first build on February 6th, 2010. Building them this way also makes it easy if you ever want to sort on the build number.
Because you're already in the tag field, it's easy to load it up with other valuable information like - TestReady, NeedsReview, etc.