She knows of, approves of, and has read fanfic (eee, how cool!). In the 'rumors' section of the site she says fans are wrong about some things:
Lily Potter still alive. (Buh? Who thought she was?)
Neville is Peter's son. (WTF?)
Lily was once a Death Eater. (What the hell are people smoking?)
In the 'news' section, she mentions that fans call Lord Voldemort 'Voldie' and she doesn't like that.
So what's missing? All those tiny little things, and one giant one (perhaps the biggest thing in the whole fandom) isn't mentioned:
When I first got to thinking about this, I was considering her lack of comment on Harry/Snape, but then I realized it was bigger than that. She's looked at fanart and fanfic, she's wandered fansites (heck, she links to some!), how could she have not encountered slash?
Yes, kids could go there, and so she probably wouldn't want to be graphic about it, but her writing on the site is mature enough that if she wanted to, she could get such disapproval out indirectly... Heck, she must know how much we fans read into every word, she could just drop some "no wizards in the HP world are gay" hint and we'd find it.
I wish there would come a time when she'd comment on slashy things, but I suppose she never will -- it'd probably be too much of a risk to certain parts of her readership. Alas!