Question for CC Makers

For those of you who've made wall hanging recolours from an artist or designers work (either in a simlish version or as-is), do you generally ask permission to use their artwork? Or do most people use the images without contacting the original creator of the image? I have a little something I'd like to make based on someone else's work, so I'd like to know what others do in this situation. I feel like asking would be best, but then I'm sort of afraid they'll just say no.