Caveat: my DS never did this much, and it might be because of something I did right or it might be because of something completely different! But based on what we did, my thought is that I would definitely let her read to herself as soon as she is ready - not teach her, but just remark to her that that's what people normally do once they are good readers. I'd also strictly limit how much I asked her to read to me - if she asks to read something, tell her you're busy but she can read to herself. That way she gets the experience which naturally leads to her reading silently to herself. (And of course, if she chooses to read fast and garbled when she's reading to herself, that's her choice.) I'd tell her that reading aloud, once someone can read well, is for special purposes: like being nice to someone by reading them a story - in which case it's important to read at "story pace" and make sure the listener can follow - or, occasionally, demonstrating that one can indeed read - in which case, it's important to get the words right - or occasionally other purposes.
(What my DS did, and indeed does, do, is to garble the occasional word he really doesn't know and can't work out - he'll say something that's acting as a placeholder pronunciation. I think that, in contrast to other kinds of garbling, is actually a reasonable strategy carrying over from when there's nobody there to ask - I remember getting all the way through The Lord of the Rings without bothering to work out how many of the names were pronounced, and it didn't do me any harm (TM) :-) When DS does this reading aloud I just tell him the pronunciation, if necessary several times until he's got it. I suppose this is a special case of working out what a word means from context, but mispronouncing it - something I did with many words, and DS also does (combine-shun for combination is proving hard to eradicate at the moment!))
