In general, when is the right time to feed new edible plants fertilizer that you've planted from seed? After they've sprouted a bit?
I take a bit of a different approach than J. Musser does (and here I am making an assumption based on the answer), but I agree with that approach when adding commercial fertilizer. Bagged fertilizer (e.g., 10-10-10) can be quite hard on young seedlings. J. Musser didn't specifically state the use of commercial fertilizer in that answer, but I believe that is the case.
On our farm we only use "organic" fertilizers. I use the quotes because I think the term is way overused and loaded and has been co-opted for marketing reasons. What I specifically mean is that on our farm I use compost, compost tea and worm castings and worm tea when I have it. I will use composted/aged manures as well (horse, donkey, goat, chicken, rabbit) and leaf mold.
In our case I am less concerned about damaging the young seedlings with these because I tend to start seeds in soil blocks or sometimes trays or small pots with a fairly neutral soil mix and then move that seedling block to a bed where I will put a handful of the compost/castings/manure/leaf mold down in the hold and put the block atop that. Later it will get side-dressed with the compost/castings/manure/leaf mold once it has been couple of weeks. I've not had any issues doing things this way and I think it is because the amendments I'm using aren't nearly as concentrated as the bagged stuff.
Again, if you're going to use commercially available mixes, I'd follow what J. Musser said. It is sound advice.
For container gardening I would leverage the power of the compost/worm tea and (limited) side-dressing with quality compost. As J. Musser points out, it is a bit more of a challenge to use the organic stuff with containers but I think that there is considerable value in compost/worm tea and in side-dressing with compost. Whereas in the garden bed, there's always room to add a little more compost/manure, in containers it is more of a challenge. Definitely consider compost/worm tea for containers. The N-P-K values will be lower than the commercial stuff but there is considerable benefit otherwise.
On most crop plants, I give them the first fertilizing at the three-leaf stage. This is when the first three leaves (not counting the cotyledons) reach full size. By this time, the root system has matured enough to handle the fertilizer, and the plant has also used up the reserves from the seed.
I generally use a lighter feeding at first maybe a 2/3's strength, to make sure I don't burn the young plants. After they shoot up, and have 6 or more leaves, I begin giving full-strength doses. So far this has worked well.