I say each campaign should have its own list of rare spells. Also higher level spells should also be of higher rarity.
Common: Basic attack magic, defence magic & spells that make life easier
Uncommon: Spells that might be used once in a while.
rare: named spells & Spells a wizard would NOT give out if they had it, like say, Feeblemind.
This of course breaks down if a PC can just up and pick any 2 spells at level up. IMG, I require prior exposure to the spells in book form for the "level up" spells, but costs are waived.