With thinning, it is kind of - the earlier, the better.
Of course, you might have something show up and eat 12 of your plants and if you have just pulled 24 and left 12 . .
. But, you can't cover all the bases. It is best to give the plants that remain in your garden a chance. A chance comes with risks.
So that they have enough light, water and plant nutrients - they need some room. Not allowing them room to grow just leads to a whole bunch of stunted plants.
Your plants will require a different amount of room depending on the time of year. Cilantro seldom requires more than about a 6 or 8 inch circle to live comfortably. Bibb lettuce probably needs more like 10 or 12 inches. These would be minimums.
If you have room elsewhere in your garden, you can transplant your lettuce seedlings. They will need plenty of water and some shade for a few days to recover from the root disturbance. You won't have very much success trying that with cilantro. It is one of those plants with a tap root and they don't transplant well.
If your thinnings are going to the kitchen, just be very careful with the plants that remain. If they are going to the compost pile, same story . . . but, even tiny plants can have use in a meal from the garden.
We are each other's harvest; we are each other's business; we are each other's magnitude and bond. ~ Gwendolyn Brooks