"The important thing is to get one that's straight enough." Paul is right -- the basic point of a strongback is to keep things properly aligned as your boat is built.
With newly-bought solid lumber (especially if obtained from a big box store), there is always the possibility that the wood will warp, bow, or twist over time, putting your T-shaped strongback out of line. There is less likelihood of this happening if you use lumber that you have had on hand for at least several months and which has stayed flat and true during that time.
I have some 2x framing lumber in my attic that generally seemed quite good when delivered -- but several months later, only a few boards stayed completely true. Most had at least a bit of twist or bowing -- not a real problem for framing purposes -- and a few looked more like egg noodles than framing stock. I put the few really good boards aside for tasks where I want something that will stay flat and true, and those boards have stayed true for a couple of years now. I also put the noodley boards aside, and made another trip to the lumber yard to get a few more boards to finish the wall framing project, where minor twist or bow was acceptable.
With plywood or other sheet material, a box girder that is true and square box can be readily made if you take the minimal care needed to rip it into pieces with parallel edges. By registering the true edges against each other, you should be able to easily build a box frame for your strongback that will remain flat and true.
However, It is not always easy to cut sheet goods accurately, especially in a shop (like mine) that is cramped for space. Having the sheet goods cut to at least rough dimensions by the lumber yard frame saw can make this task easier. But especially in big box stores, the folks who make those cuts often don’t get the dimensions just right -- although the saw itself generally produces good, straight, parallel cuts.
