i use the regional rate boxes all the time...sometimes it allows me to make several dollars extra profit...i will offer calculated shipping, or charge for a medium flat rate box on my listing...then once the item is sold if my customer happens to be closer to me i can use a regional rate a or b box and save several dollars...

just remember the regional a is good for 15 pounds and the b is good for 20 pounds...if you go over on weight and they catch it your customer will not be happy