See post above yours, best is always best, of course, but is it absolutely necessary to have best all the time, that first show anyway?
If so, request it, trainer will try to accommodate.
Every time until the horse is seasoned, maybe not always necessary, as the difference for an owner’s horse, unless a top horse in a top field, like hoping for the best jockey in a big race for your really good horse?
Yes, is best.
Is it terrible wrong for the top trainer, not a designated appropriate assistant to ride some trainer designated horses for that assistant?
If the trainer looks at that horse and thinks, hmm, I better give him his first few rides in the shows.
If he looks at that horse and thinks, he would be perfect for my assistant to show, both are ready for that level of riding and training and they make a great pair.
That would be a difference if to present to an owner a choice, or explain to the owner that the situation would be good for horse and assistant.
Those situations come up regularly with all horses a trainer has and how to manage their barns.
That is why I think the answer here is, it depends.