Some things to consider:
Most TTs use lug centric rims. The center hole is not necessairly in the center of the rim. This means that if the shop uses the center pilot hole to mount the rim in the balancer, that it cannot balance the tire. In fact, it is very possible that balancing it that way will make it worse.
Most, if not all trailer drums are not balanced at all. So balancing the tires won't really gain you much there.
There is a on the vehicle balancing system out there. I have never seen one, so I think they are kind of rare... But if you use this system, then the it will be balanced no matter what rims and drums you have. The downside is that you cannot rotate or change a tire, or even remove it to service the bearings without making sure that it goes back on the studs in the exact position it came off from.
And then there is the percieved benefits.
The most common is better tread wear... But most TTs will never wear the tread out on a set of tires. They age out first. There is some thinking that there is a slight benefit to wheel bearing life. I doubt that it is enough to have a huge impact on the cost/benfit ratio though.
I did the math on this many years ago, and figured that forgoing paying for balancing over the life of a TT will easily add up to enough for a set of tires.
Many years ago when money was tight, A tire shop suggested to me that I could forgo balancing on the rear of my truck. He said that on a solid non steering axle, that I wouldn't notice any vibration. He was right.
Our TTs have solid, non steering axles.
I haven't paid to balance a TT tire ever. Often it is thrown in, after the price has already been negotiated down as low as possible... Then I ask if they will do that to sweeten the deal. They usually will... Maybe it is because I buy expensive tires though, and they don't want that sale to walk away. Oh yeah.. Don't be afraid to walk away. I have had them call me 20 minutes after I left with a much more generous offer than they were willing to do before I walked.
