Culling is a very difficult process. All sorts of things enter into making the decision to keep or cull.
It's difficult to judge whether the first blooming seeding is either immature and genetically predisposed to bloom or if it's mature and is a poor specimen. I've settled into a two season evaluation period before making a final cull.
If the majority of the plants are blooming and all seem to be growing well, then I cull at first bloom. I don't like poor color, poor form, poor opening, weak scapes or low bud counts without spacing. I'll tolerate stiff foliage and short scapes if the flower is distinctive. I mark all first year exceptional seedlings to compare the following year. At the end of the second season, those selected seedlings go into an evaluation bed with more room to develop into clumps for overall evaluation. My early seedlings are too crowded to evaluate the entire plant. After a couple more years if the plant looks good, line it out to see how fast it recovers and plant it in different spots or places.
You also should give some thought to protecting exceptional seedlings, such as a good friend or relative. Things happen, and sometimes promising seedlings are lost. Sharing some with someone that lives in the rust belt would be good, if you have plans for eventually selling the plant. Resistance to rust is a good selling point.
Good luck, have fun! Ed

Thanks Ed,
I appreciate your insight. I have no plans to sell, just for my own pleasure, but like you I have a problem with not enough space and need to get rid of some.
FFO sdling

Your foliage and scapes look mature. Heavy substance, highly ruffled flowers take higher temps and adequate water to open well. The second flower you post above, I would probably give extra chances to please me. If it starts opening decently, it might at least be a good bridge plant.
It's very hard to appropriate space. As I said, my seedlings are rotated on a 3 year basis and I reserve 1 bed for selected seedlings evaluation. I go through my collected seeds each season and try to start only as many as the seedling bed will accommodate. Also keep an eye on how much energy you want to devote to this pastime. If it becomes too much work, it will cease to be fun. There's always obstacles to overcome and the more plants you have, the more likely they will happen.
Good luck, Ed