Put them out. Lettuce should go out prior to your last frost date. In my zone 5 April 15th or even a bit earlier is when it is usually safe to put lettuce out so you should be close in your zone as well. The lettuce can usually take light freezes just fine. I have even seen it turn to mush and bounce right back when it warmed up.

Here is a bit more information:
My last frost date is May 15th. The ground is still frozen and I have snow in some areas as well. It consistently drops into the twenties at night. So putting them out is not an option right now.
Due to my work schedule, the earliest they are going outside, whether they like it or not, is May 6th. My dileama is what to do until that point in time.
That is good to know that lettuce is so hardy! Thanks again!

Yikes, don't be digging in that frozen ground :)

I don't think your lettuce seedlings look especially rootbound.

Floppy soft growth may be from your artificial lighting and they will probably become more sturdy when introduced to natural light...also too warm, or overwatering can lead to soft growth.

Linked, pic of lettuce seedlings with adequate roots, but not too rootbound or in immediate need of potting up...

Here is a link that might be useful: {{gwi:1056166}}

thanks morz8, a picture is worth 1000 words!