I would say if your skill is at 6, there's not a whole lot gained by plinking away at raising it much further, especially given Limits. For magical characters, the best use of karma is initiation, Magic increase, and/or gaining more spells. Same for TMs with their Resonance stuff. Now, for mundanes, at a certain point you probably have more than enough 1-2 rank skills to round yourself out and probably will start turning those 6s into 7s etc, but that's more out of a lack of anything else to do with the karma aside from buying Edge.