It will depend on the size of the rootball that comes with them. Most upstream branching occurs in the first 4" before the prune. Rootmakers are a great idea if you are getting trees you can't get in the ground this year. For example, if you got bare root trees in late spring, hot weather is approaching, and you can't provide supplemental water in the field. Instead of planting them in the field, it would be reasonable to put them in rootmakers and care for them at your home over the summer.
The beauty of rootmakers is the system of containers. 18's prune the tap root, and when they are full, I transplant to 1 gal and then 3 gal RB2s. This moving from container to container each roughly 4" larger than the previous maximizes the root system. Because they are pruned at the tip, no energy is lost. When we prune by hand, we are discarding tissue that required energy to build.
So, don't expect the same results from putting a tree that is already 2- 4 feet tall into a root maker. Much will depend on the trees you get. I did buy a few AU Buck chestnuts from the wildlife group and a few seguins as well. They were between 2 and 4 feet tall when I got them. I put them directly into 3 gal RB2s. Keep in mind that they were actually grown in roottrapper bags before I got them, so the root system was already dense.
I don't know if your trees are container grown or bare root, or B&B. I've also purchased apple rootstock, grafted them, and grown them for a season on my deck in rootmakers. The size of the first rootmaker depended on the size of the rootstock. They don't have a tap root because they are clonal. They have such week root systems to start with, I need a container large enough to fit all the roots as I don't want to lose any.
If your trees were grown and shipped in smooth containers, you'll need to check them for a circling or j-hooking roots and prune them by hand. If they were grown in the ground and dug up, the spade they used to extract them will have pruned the roots.
You will just need to see what you get and then decided how to handle them.
Thanks,
Jack