Depends on what kind of target shooting you're doing... Mirage could indeed be a problem at extended ranges and it would be handy to have a variable power scope that does have some higher magnification but the ability to zoom down to a lower level if mirage becomes a problem.
I'd consider something like a 6-24x or 8-32x variable power scope to give you a wide range of magnification.
I have a Leupold 36x scope on a .22 that I use exclusively for benchrest shooting at 50 yards, but thats because of the level of precision required.
It's pretty impractical for anything but very precise benchrest shooting, but it works extremely well for that.