Obviously having an 9.8 sketch is the preference, but I take into consideration many factors such as the quality of the sketch, the artist, style etc. . Some of my favorite sketch covers in my collection are not 9.8s.
You're correct that these sketch covers have exploded in popularity over the last few years and I think there are many collectors that appreciate it, except for those die hard OA collectors. I understand some OA collectors look down on sketch covers, but some sketch covers by some artists rival 11x14 OA commissions. IMO.