I often see, and as often doubt the value of, one particular question at the end of forms for in-house lawyers to evaluate the firms they use: “Overall, how would you rate the firm?”(See my post of Nov. 16, 2005: evaluations of law firms with 9 references.).
If you have asked direct and comprehensive questions about manifestations of firm performance, you can easily compile those responses and create an overall rating. To ask for an additional, all-in rating is over-kill. Furthermore, such a grand appraisal hardly seems very reliable as it must be an agglomeration of disparate impressions. Better, in my opinion, to analyze the more specific ratings and even weight them.