Very low varroa mite drop with mine this year, too, and none found in sacrificed drone brood. However, the little monsters can build up so fast that I prefer to keep them below a level where I can see them at all, if possible. So I'll treat this autumn, but may or may not do anything mid-winter depending on the autumn results.
It's easy to underestimate the effects of varroa - I've known really experienced beekeepers get caught out and lose hives as a result. Also, it seems to me that mite drop rates can be deceptive: from looking at drone brood - which was quite badly affected early this year - I'd say that something is doing a good job of clearing up dropped mites, because from the drop rate alone I wouldn't have suspected a problem. I hate destroying brood but I haven't yet found a better way of keeping track of varroa.
