I had a hard time separating true, physical incest from emotional incest and just plain squicky (or impossible - see Ben/Glory, Fred/Illyria, etc). I think I only gave 10's to the truly incestuous, but I probably included at least one emotionally incestuous one there. I tried to confine my squicks to the 7 or 8 range. Spike and Dawn squicks me as a concept, although I have read a few very good, very believable dark fics in which it works. Still squicks me, but I could read it because it was so real. Older Dawn and Spike somewhere down the line - yeah, I could see that. Rather not, but I can. I think I even wrote a drabble once that implies it. Spike/Joyce falls into the same category as Giles/Willow/Xander stuff. Not remotely interested in it, and it isn't the relationship they had in canon, but it doesn't particularly make me shudder.
Oddly enough (in light of comments I didn't see till after I voted), the whole vampire family thing being incestuous didn't occur to me at all. In canon, the sires do have sexual relationships with their offspring. It's part of the package. So I didn't see those as incestuous at all. Hmmmmm