I've been making full HQ gear sets for my retainers but in looking, I've not been able to find how much of a difference it really makes or if it's even beneficial to replace my retainer's gear every 3-5 levels with the best HQ stuff.

Both my retainers are Archers, as I found it easier to make the gear for two identical classes. I also felt it was safe to assume that in terms of venture rewards, DoW would recieve the same rewards no matter what class they were.