I spent about 4 or so years on my TB not doing any warm-up jumps before heading into our 1.20m - 1.50m classes. The footing change at our primary venue (GGT-ish mix in the schooling, grass in the show ring) wasn’t conducive to a productive school, and it took me several shows to figure out why he was pulling a couple of rails early on and then jumping clean for the rest of the class. Cut out the warm-up and we went back to a much higher percentage of clean rounds. It didn’t help the matter that he was a huge spaz in the ring with other horses. If someone knocked down a jump when we were anywhere in the vicinity he would lose it.
So for the first couple of years (1.20m - 1.30m) I wouldn’t warm him up at all (in a traditional sense). Instead I would take him for a 30-40 minute walk around the showgrounds and then head straight into the ring.
Then we started spending most of the summer at one showground in particular with the footing difference mentioned above. For those couple of years, I would walk up to a big grass field overlooking the GP field and trot and canter for about 20 minutes before the class and then head straight in. Scared me almost to death to not warm up (especially after watching the big guns jump a lot of fences in warm-up, because seriously, who am piddly little I to question what 99% of the show world does to get ready for a class?!), but it worked beautifully for my horse.
Then they changed the footing in the warm-up ring to a different GGT blend and I made a go of schooling again (for my own sake), and now I’m back to jumping 5-6 fences before walking into the ring. I do always make sure to canter a bit on grass and do a couple of halts before we go into the ring (I still think it’s a tough transition for my guy to go from footing -> grass).
For my lower level youngsters, I also often do not do jumps. I figure pretty much anything below 1.0m is fair game to walk right in after a good flat warmup. So if I’m having trouble finding a fence setter (occasionally the case when I’m at a show by myself without a helper), I just don’t sweat it.