Probably depends on the variety but they may all be bitter if it started bolting some time ago. You can sometimes sweeten up the leaves by soaking them in ice water.

It also depends on your tolerance of bitterness in lettuce. Personally I like it. You can also still cook bolted lettuce.

I pick spinach up until the leaves are too skinny and pointed to bother with, never got bitter spinach here. Lettuce, oh yeah, it's the main reason my spring lettuce never kisses my summer tomatoes, lovers passing in the night.