I have two experiences with upward fixation of the patella. The first is my riding horse that last winter did an exuburant buck and fall on a lunge line that resulted in her hyperextending her right rear leg ending with upward fixation of her patella. It was so bad immediately that I thought she broke her leg!! I did all the hill work I had available to me which improved the problem at least 80% but there was still a little “hitch” so I had her blistered last summer. Took two more months of hill work and now she is comletely fine. I work with a highly respected board certified equine surgeon and blistering a stifle is his last effort protocol to helping resolve the problem.
The second one is my just turned 3-year old that off and on during his 2-3 year old years has had upward fixation of both his left and right patellas. This boy is super tall (16.2+ at 3 years old) and the vet feels it is all related to rapid growth and not being worked/ fit yet. He sees this commonly in a tall rapid growing horse and the vast majority of them resolve once they level out and beocme fit. This gelding is just getting started under saddle but I pony him on the hills at least 2X/ week. His stifles have improved at least 80% as well with just this limited amount of hill work. FWIW every person that has contacted me about the above horse (as he is for sale) I have been up front about this and send them the link below. If it comes to the point of him needing his stifles blistered I will consider it, but only after a serious fitness plan on the hills is executed and consultation with my vet. I would be absolutely forth coming with this information to a potential buyer. Like the OP’s horse it has thus far had no bearing on him moving forward (he is a lovely mover)…you only notice it if he trys to back up or lift a leg to pick out a foot…and only intermittently at that.
Having said all that and to answer the OP direct question…there should be no need to blister the stifles of every youngster. Yes, upward fixation of the patella is a common growth issue of larger horses but it should resolve as the horse matures and starts under saddle work. An exercise plan of hill work should be performed before any invasive procedure such as blistering. However, if that does not resolve the problem blistering can be an effective procedure to resolve the issue and lead to long term soundess.
Here are some really good links:
http://www.atlantaequine.com/pages/client_lib_UpPatellerFix.html
http://www.atlantaequine.com/pages/client_lib_IUPF.html
http://www.atlantaequine.com/pages/client_lib_PostBlisteringReco.html
http://www.atlantaequine.com/pages/fitness_training_sched.html