For example, if I do straight-leg DL for the back, do I then do a hamstring exercise also? Or if I do it for the hamstrings, do I then do a lower back exercise also?
Straight-leg deadlifts are listed among the exercises for both erectors
, so you're killing two birds with one stone. Ah, the beauty of compound movements!
As for regular deadlifts, I share your puzzlement. On this site it claims that they work the quads, glutes and erectors, with the hamstrings as dynamic stabilizers
. But on most other sites, deadlifts are viewed as exercising the hamstrings
rather than the quads.
Maybe you'll get your answer as to which muscles were worked based on what's sore tomorrow. ;-)