Depending on how many you’ve used already (say on Glade Warden), at least 4 are (currently) useless because there are (currently) only 6 Common troops needing one each. I happened to know this one, but for every other traitstone, this guide from KAYA43V3R has the totals.
However, in March, the Spring Imp will be along, needing 18 of these babies.
Nevertheless, it would be handy to have tables for each Traitstone showing which troops need how many of each, so you know which ones to save up and which ones you can freely spend on boosting Kingdom Power.