I grew lettuces under lights last year, LED and T5HO's. They were very short, bushy and wide, and would have been too crowded in a 6" pot for more then one. Very different growth pattern then I get outdoors. If you are planning to grow in a window, plants tend to be taller and thinner and it may be a different story.

Thank you for your reponse. At this point, I don't need to worry about thinning, because both the lettuce and the spinach died soon after sprouting.