Gee...Gee, it depends on what you are planting. Some people plant their hardy perennials as soon as they have their 2nd set of leaves...their true leaves. I find if I plant them that small, I am liable to lose them. Either I'll weed them or the critters seem to get them. LOL I wait until they are larger. I also make sure to wait until the ground is ready for digging in the spring. If you dig too soon, you (in unscientific terms) wreck the structure of the soil.

If you are planting out annuals, you will find they are hardier than seedlings grown inside, but I still wait until the last frost date, unless I plan on protecting them.

Different people will have different methods, and you do have to experiment and finds what works for you. Good luck!

Guiding rule of thumb is to transplant the seedlings once they have their second set of true leaves. The first leaves you see when the seeds germinate are cotyledon or "seed" leaves. The next leaf or pair of leaves are the first "true" leaves and will generally resemble the leaf shape of the adult plant.

This was my very first WS sprout:
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Most WS hardy perennials aren't bothered by cold nights because they were grown in whatever conditions Ma Nature threw at them and are acclimated to weather extremes. They sprout when the amount of light, the temperature & moisture triggers their genetic code that the time has come to germinate. Annuals, vegetables and half-hardy/tender perennials will need to be protected from frost/freezes.

ditto on second set of true leaves. Follow the lead of the seeds - they know what to do.