Sanitation wise, it *should* be fine, but boiling for a few minutes at the end of the boil just assures that sanitation is good. In either case, adding with five minutes to go or at flameout, you'll want to pay special attention to not scorching at the bottom of the boil kettle. Its very easy to scorch. I take the pan off the flame, take some of the hot liquid and put it into another pan and start mixing the extract into it, stirring like a mad man, basically thinning it before adding it back into the boil kettle, stir more, then back onto the flame.