After 25 + years, I have learned to turn off some of the stage manager in me and treat a first read through of a play I'll be working on as an "overall, mostly, what is the playwright saying?" read. Keeping note paper and pencils away from reach is also what I do to keep from going into SM mode. After the first read---it's all business. I do like to read the play before the first read thru with actors and that's mostly because I'm often invovled in auditions. It's easier to confir with the director if I am familiar with the characters/play.
I also read plays that the theatres/univeristy I work with may want to consider as part of their season. Those reads are 1/2 pleasure and 1/2 "is this really do-able at this place?"
What a great topic!