Hey, there are hard-and-fast rules about what's "acceptable." You have to decide for yourself if a given song in a given capoed-way-up-the-neck key a) is playable and (b) sounds OK.

My feeling is that a guitar choked up to the ninth or tenth fret sounds a bit thin by itself, but can add a nice interesting sound when played along with another instrument or two, like a standard-tuned guitar with no capo or maybe capoed at the 2d or 3d fret. Just last week, some old friends kicked off a song I normally play in D, but they were in the key of C. I clamped that capo onto the tenth fret, scrunched up my left hand as small as I could, and proceeded to pick ~ sounded great!