It depends where you are located. If you are in zone 5 or higher, the lettuce season is about over and you might as well cut the whole head before it bolts. However, if you live in a colder climate, you may be able to just pick the leaves and then allow more to grow.
When you say tang, do you mean bitter. Sounds like the heat has gotten to it. I think you may have waited to long. You will see even more milk substance when it bolts.