How far out of square is too far?
I would say it's a little more than what's normally considered OK, but it isn't going to make the shed collapse or anything like that.  It's going to make for gaps in the corners, trim might not fit quite right, stuff like that, but probably not an issue for a home built shed.  

I would weigh the aggravation of fixing it against the aggravation of working around it, and correct it if I could without major surgery.