Dimensional treated yellow pine lumber, non-standard widths?
Reorder the beams so they are installed from shortest to tallest, with same heights together in sequence.  I would place the tallest next to the building.

With a little luck, a board across them will create a straight line.  If it does not, how much variation could there be?  1/16th inch?  1/32nd?  If you are blessed with perfect climate which will not distort the lumber's dimensions, then maybe some sliver shims are called for.  Otherwise, prohibit your guests from laying down and gazing across the decking.