Well that is a very subjective question which would depend on what spells you chose the first time and what spells you plan to get afterwards.
I thought about doing it for my Ice wizard because I trained him in spells from 3 or 4 different schools but without any higher level spells. I was thinking it would be nice to get feint from death and Satyr from the life school which would help him out a lot. Eventually I decided against it because of the cost in crowns and because I could just use treasure cards for those 2 spells. I also have other wizards and don't play my Ice as often as I used too.