I know what you mean
I might correct spelling, put a -0, etc at the top, but with first graders i am often unsure because I don't think mirror writing is incorrect, but some might...
Pretty much I do not ever do any correcting at all. If I have any down time I am usually reading something that I have to teach next, or taking a BREAK I feel that we don't get paid enough to be doing it all. Also when the bell rings, I dismiss the kids and then leave myself ASAP, even in schools that think subs should stay an hour or more after the bell. forget that!
I do, however, always leave the room very tidy, pick up anything off of the floor, wipe down the sink area, and leave a short note for the teacher. ;-)