I've got my own balancer, and mounting machine, because I do a lot of racing and go through a lot of tires. I keep an eye on my street tires' treadwear, and when I see the fronts starting to wear down, I rotate front-back, and keep going. since there's SOME event every weekend, it's no extra time, just rotate when taking the race wheels off.
I won't usually check balance unless I notice something funny in the wear of a tire, or if I feel an imbalance, but I tend to go through them fast enough that it's not an issue for me.
Depending on the type of tire you have, and the tread compound, and the type of driving you do, you can have noticeable imbalance (generally, most people won't notice anything less than what a 1oz weight would fix) in as little as 50 miles, or not have to re-balance for the life of the tire.
sure, manufacturing tolerances on tires and wheels have improved, but so have suspensions and steering systems, so you can feel the imbalances much more, especially on a vehicle that has "performance" suspension. If you're driving a 1-ton truck with tall sidewalls and lots of suspension travel, or a land yacht with cushy soft suspension, you're not going to notice the bumps in the road. Yet driving the exact same road in, say, an '85 Corolla with track-tuned suspension, you will get the teeth rattled out of your head, unless you're going over 70mph. (but, that gets into motion ratio and more suspension tech, out of the realm of the tire balancing discussion...)
If you bought tires from the shop along with lifetime balancing/rotating, USE it whenever you want, because it WILL make your tires last longer, and save you money in the long run. Just because you can't FEEL the imbalance, doesn't mean there isn't one, and there goes 1/32nd of tread before you know it. Quite a bit, when you only have 11/32 to start with....
--sarge