I can train to 134, so I can’t check… does the dragon info not show the stat changes if you can’t train to the next level yet? I would think if that shows a “+X”, you can safely assume training won’t make the stats worse.
That said, FWIW I usually don’t alternate training dragons and using them as collectors… when I decide to train one, I keep training it until max level before starting to use it as a collector. If you follow this, it won’t matter what the stats are at 125-133.
(Incidentally, I’ve retired Skystreaker as a collector, as I have enough dragons these days that have better collection rates… Bargemine, Grazefall, Bonecrusher’s Conquest, Sinker-Claws, Sven’s Nightmare, Ripwrecker, Threadtail, Scardian, Twist-Trapper, Rushing Death, Sirenade, Sparkle&Sparks… Basically, I’m not using anyone under 12M rate any more. Skystreaker is a good runner-up though until you have enough over-12M to use up all your slots.)