I don't think when you use it matters. All 16 games count the same. The non-season-ending injuries aren't particularly clustered anywhere.
I think the way to maximize the use of the return IR is to use it on a guy whose injury takes a full 6 weeks to heal. They are allowed to practice after 6 weeks, then play in games two weeks after that. Tretter's injury probably fits that time window well.
I wonder if Linsley will be better or worse than EDS from last season.