Last spring, as part of a discussion of traditional New England trim in JLC Online's finish-carpentry forum (search on "New England home details"), Jim Chestnut posted a great explanation of the benefits of the traditional jack-mitered beaded casing joint (1, 2):
"Beaded casing is an excellent choice, with or without plinths or backband. Its biggest advantage is versatility. If you use a backband, the bead can be jack-mitered, the rest of the casing butted, and the backband mitered. If you wish, the jack-mitered legs can be pocket-screwed together before installation. You'll have no problems with wood...
to read the full article.