I probably differ with a lot of people here, but I keep my routines really simple and really basic, with usually only one (MAYBE two) excercises done per bodypart each time I hit that bodypart.
I don’t consider DLs to be a “back” excercise in the way I consider rows or pullups to be a back excercise, as those excercises hit your lats very heavily (other things too, certainly, but mostly lats). DLs hit PLENTY of other areas of what is truly your back, but not so much your lats. So I don’t look at them as part of my “back” (i.e. lat) routine.
I personally use them interchangeably with squats. I know that one is technically quad-dominant and one is hip-dominant, as Ian King would say, but I find that DLs hit the living hell out of quads too, almost to the extent that squats do, as well as hitting things that squats don’t (hams, lower back, mid-back – though not really lats --, upper back, traps, etc.).
I do one or the other (squats or DLs) once per week. If I did squats last week, I’ll do DLs this week, and so on. This has served me incredibly well, and has kept things nicely balanced.