I have been experimenting with this a bit, too. I put a few in soil right after their roots appeared, but for others have waited until the cotyledons grow out a bit before moving them into starter mix. Yes, you have to be careful when putting them into the soil when you do it this way, but I've found it informative and even a bit entertaining. For example, Saturday evening I pulled a sprout out of the ziploc, and it was kind of folded up like a letter 'v', which frankly made putting it in soil a bit tricky (note they don't all do this.) Put it in the light all day yesterday, and it straightened up nicely in almost no time at all. I have the light to the side, and clearly need to change it to directly above, as by noon today the little one was bent almost clean over towards the light. Turned it away at that point, and found it bending back towards the light again later in the afternoon. While plants do this naturally, regardless, I'm finding the new sprouts do this very dramatically in their first few days - they will firm up and stay straighter in about a week. Also, I don't have a fan directly blowing on them, but lightly flick them a few times a day, and it is interesting to notice how the stems are firming up over just a day or two.