I start with forms upright. A string from tip to tip gives a perfect reference line. With the forms hull-side up, any string you try to run will be touching all of the forms, which can affect how straight the line is, so it's not very reliable (of course, a laser line would solve that). And don't forget to sight along all the other reference lines that are printed onto the form drawings, like waterline, etc., as these are just as important and they can be off even if the centerlines along top and bottom seem aligned. As long as your forms are firmly mounted to your strongback, and you are careful turning the forms over, there shouldn't be any problem with the forms moving on you.

For an internal strongback, I took the extra precaution of preventing my strongback (a 2x4) from twisting or warping by starting with the equivalent of an external strongback--a 16 foot length of 2x12-- and mounting onto it three plywood (MDO)brackets with holes cut into them to snugly fit the 2x4 strongback. These brackets were perfectly aligned in all three dimensions, then firmly mounted to the 2x12, then I just dropped the 2x4 strongback into the openings in these three brackets. Doesn't show too well here, but here's a pic anyway.

Roger T.
Orr's Island, ME