When you purchase nursery stock do you re-pot right away with bonsai mix, or wait until the re-potting season?
It depends on when you purchased it, and when it was put in the pot.
Safest is to leave it in the pot till the next spring.
You can carefully pull it from the pot and see if the roots have set in. If the roots are set in or worse yet root bound it is safe to repot. Even then, especially for conifers, you should wait until the optimal time for your specie of tree to be repotted. Remember to not bare root conifers. You can use this time to build vigor in the material. Some even do their styling in the nursery container. It all depends on what you want to accomplish and the health of the tree.
As far as the repot mix, some say you should use a transition mix, and some go straight to aggregate mixes.
There really is not a repotting season. Yes, most repotting is done some time in the spring. Find when it is best for your material.