"Not that expensive to place" is the exact reason you WANT to reduce their cost. They'll be even cheaper.

I will put it like this: The best thing to do is to save her for now, unless you have a -2CR black around.

What people tend to do is to get a black to -2CR with either another black or the black CR fairies (we will get them from collection events later so keep onto that Fleur if you do not have any -2CR blacks yet). This is because the black CR fairies have the same chance as fusing a same copy black, meaning that last chance will be 25%. Fleur however is always 100% so using her for that last CR is the best.

So, yeah, i recommend saving her. Myself i used it on my already -2CR Mikoto to make her mincosted.

There is little point in CRing stuff like Cornelia, Deine and the other high costs. It is not often that you need to place them early and it really is with the low cost units that the CR matters the most since they have a bigger impact on the early game.