Initially lining up the edges of two boards for gluing up a jig is pretty easy. But when you add clamps or screws, one board often shifts out of place.
If that alignment is critical, I don’t use clamps at all and divide the job into two steps. First, I apply glue and rub the pieces together, using firm pressure. When they’re perfectly positioned, I walk away. About a half hour later, I add the screws. Of course, nothing moves then!