It was straight as an arrow out of the box. (the screws you see aren't causing this bend, they don't hit the ground ... they're just holding the drill template for my T-Track linear rail mod) The T-Track, thankfully, pulled my platform back into shape.
My thinking here is that the belt pulls slightly downward at either end. The guides act as a fulcrum to create what is essentially a bender ... and not at all amusing like the one in Futurama. I added an extra nut to keep the belt even along the x-plane. Also, if you attached your belt as shown in the build manual, the belt pulls a few degrees in the y direction, accounting for the bed wobble. Wrap the belt the other way around so the smooth sides of the belt meet up. That is, looking down at the top of the platform: clockwise on the left, anti-clockwise on the right.
...speaking of belts... The gantry belt likewise pulls at an angle. I moved the pulley up a bit and added a deflection roller to correct it:
(The motor-side is slightly off because I used the lead-screw cap as a temporary roller ...slipped it over an 8mm-OD bearing; I've since printed a proper sized roller).