When to Move Plants Outdoors

Thank you so much for your detailed answer to my question. I'm sure after I experiment for a few years, I will get a better idea of the best timing for my particular microclimate. One general question first. At what stage should plants be before they are moved outdoors (assuming the weather is co-operating)? Should they have several leaves or are they hardy enough when just the cotyledons have emerged?
The seeds that I purchased this year are Florence Fennel, Greek Bush Basil, Leaf Dill, Caraway and Sea Holly. (I live near Midland, Ontario in Zone 5a (or 4b depending on the source) and the plants will be going into a south facing bed near the house.)

Cotyledons are the first set of leaves a seed produces. In most plants, the cotyledons are followed by the first set of true leaves – leaves with the characteristic shape of the leaves of the mature plant.
Seedlings at the cotyledon stage, before the first true leaves form, are much too tender to to be planted outdoors. Seedlings from the cotyledon stage up till the fourth or fifth set of true leaves, can be transplanted into pots for further growth until the plants have reached a size where the roots are well established in the pot. Typically, the roots must be dense enough to have formed a good solid root ball that will not break apart easily during transplanting to the garden. By this time, the young plants may have 10 sets of true leaves or more.