Bolting and bitterness aren't necessarily tied: I've had beautiful big
heads of "Red Sails", nasty bitter and no sign of bolting and I've had
ones more than starting to bolt-like 18" high and hardly bitter at all.
My conclusion is that lettuce likes even moisture, in the soil and in
the atmosphere. If you get hot, dry, windy, lotsa sun, watch out:
bitterness is likely to follow. As someone has pointed out, some
lettuces do better in summer conditions. For a great selection of
lettuces, spring, summer, fall and winter check out Cook's Garden
(www.cooksgarden.com).