I think it's OK when they have a set of true leaves.

Yes, one set of true leaves or more is the general rule of thumb. Remember to handle very gently, and to hold the seedlings by the leaves, as the stems and roots are more susceptible to bruising. You can usually tease the roots apart gently.

They are alot tougher than they look. And teasing them apart is easier when they are little. The roots aren't as tangled as when they get bigger.

How do you know if they are true leaves?

First you generally get 2 leaves when the seedling sprouts. The next two leaves and the leaves after are true leaves.

The first leaves that show up are cotyledon leaves. You can Google how to say it but it's easier than I thought. Mostly they're oval round or oval elongated. The next leaves that appear should look like the "true" leaves the plant will have as an adult, only in miniature. For example, lady's mantle leaves are round with a scalloped edge. The seed leaves are oval just like most others. The next leaves that appear are round with a scalloped edge.
I've discovered these WS sprouts have amazing roots so I just dig straight down to the bottom of the container with a soup spoon to be sure I get as much root as possible. Then I just gently lift the clump of soil upwards and press it firmly but gently into the soil of the next container or the ground. Goes without saying the hole in either the pot or the ground is already dug, filled with water and ready to receive the seedling.
Like clc70 said, they're tougher than they look but you're not handling a mature plant so care is essential.
Happy planting!!!