Initially, no more than 6', because that will raise the spoilers.
Wrong! Mr Boeing has taken the use of the spoilers into his performance calculations for takeoff. Use as much roll control as you need to keep the wings level during a crosswind takeoff. The 'negligible drag' comments are also red herrings - whilst there might be some drag the major factor being considered is actually the generation of lift! But as I said, this had already been accounted for in the aircraft's certification.