Anyway, back to sort of the original question: Getting your tires balanced at 10 or 20 thousand instead of at every 5000-mile rotation is not like neglecting an oil change that long. If they're not displaying a vibration or a "shimmy" they're probably alright. (Spots of excessive wear in the tread can also be a symptom, but I guess that you have to ignore a lot of other symptoms before that one shows up!) A lot of people don't even do routine balancing, just when a tire is first installed or is repaired, or symptoms of imbalance show up.
Incidentally, it helps to know where your wheelweights are supposed to be (note that some of them can be on the inside of the rim). Ever notice how many wheelweights you find in the gutter? They all came from somewhere... A discreet mark made with a pencil or a magic marker, or a digital camera photo, can lend some weight (sorry...) to a suspicion that there is a vibration, or at least convince the tire shop that you suffer from obsessive compulsive maintenance disorder, possibly inspiring them to more-careful work.
--Joe