Even though it may be frowned upon by most pro builders I've found that a little rear dropout filing and filling can help correct rear end build errors. This assumes you have suitably beefy dropouts of course. File one side of the slot and fill the other as needed so the axle will be held tight. I don't recommend going more than 1mm or so using this technique but that will buy you a lot of alignment correction at the tire end of the wheel.

And BTW, if you aren't set up to check alignment of the rear end using your surface plate the string test is a pretty good ghetto test. A thinner string than shown and align off the face of the dropout is recommended. Use something more accurate than a tape measure to measure too.