Sure, radial force variation can change as the tires wear. It all depends on how the materials within the sidewall "wear in" with mileage. That's why some peeps will rebalance new tires after 500 miles or so in order to give the sidewalls an opportunity to break-in a bit and get a more consistent balance. Air pressure also plays a small role as well, I'd imagine...as in, if the tire was RF balanced at 36psi, the "balance" may change if the tire ends up losing air, etc.
What's also important to consider is that tires can often flat spot, even just overnight. Therefore getting the tires hot by driving on them for a while THEN RF balancing is probably the preferred method of keeping things consistent.
Yeah, before I got a handle on this whole RF notion, I found a place that does on-car balancing too. I was at my whits end at that point and figured that was my only remaining option to cure my vibration issue, but never had the opportunity to try them out given that I eventually found a competent shop with a RF balancer that knew what they were doing.