Molly -- I dont know if i consider myself "experienced" ... but usually the first things i plant out are the Poppies. they may go out next week in between rainy days. [that way i dont have to water them]
most every thing else starts going out in May. I've gotten lucky and planted out my Tomatoes the first week of May, but i did add a bit of protection by placing a halfa 2ltr bottle over it on windy days.
I've never done lettuce, so can't really help ya there.
I'd think if they are quite hardy, they can go out... you are warmer than me.
