I noticed a bit of that problem when building my 2 Arctic Terns. It seemed to be caused by a variation in stiffness between one panel and another.
IF, however, you're talking about twist, as in the broomstick example, that's very important to remove.
1. If you haven't epoxied all the seams just yet, inspect all of them to make sure they're stitched edge to edge with no variation along the length of the seam and adjust accordingly.
2. Instead of the broomsticks, I used two straight lengths of 2 x4 across the sheers with a carpenters level on each. That tells you how much twist you have without visual error thrown in.
3. After making certain there's no twist , hang a string with a small weight from the tip of the hull and stern to check that both seams are vertical, adjust accordingly.
3. If one side of the hull looks a bit more curved than the other, it may just be variation in stiffness between the panels. If it's not out too much, say less than .5" then that asymetry will be removed when you attach the deck.
4. When I joined the deck to the hull, I stitched them together instead of trusting that packing tape would hold everything in position. That made it much easier to get both into alignment for gluing.