as a general rule of thumb, you'll usually prefer using stones on non wild pets because they can't be upgraded by any other means. however, if you don't plan on levelling any of them soon, upping a rare pet with the ideal breed (!) isn't a stupid thing to do either, if you're going to level it anyways. i'm saying this because i've been farming 4 hours for a rare breed 5 rapana whelk, which is a common spawn. who knows how hard rare top breed rare spawn wild pets are to come by. i most certainely don't have the time for such follys.