Upon reflection, maybe the line between "healthy" and "unhealthy" has more to do with parental expectations of performance (or behavior) than it does with anything to do with level, time, timing, or how child-led things are.
Of course, I think that what we did was healthy. This may be some selection bias on my part. I'm sure that we made other mistakes to make up for that, though. LOL.
We had no particular expectation of proficiency or performance beyond the informal 'reading' sessions with the specific phonics books each afternoon. DD was able to decide what use she wanted to make of it all, and that was fine with us. It would probably have been healthier to have done it when she was just three and first expressed serious interest in learning to really read. We instead relied upon a knee-jerk reaction that only pushy parents "taught" their children to read that young, that there must be something harmful about it, and that we weren't going to be among "those" parents...
We went through the same thing with toilet training. She was just ready so early and we were still so stuck on ND expectations that we didn't believe it, I guess, and refused to cooperate or provide tools during the window of greatest intrinsic motivation on her part.
Hindsight is 20/20, however.