Personally, I always wait until official WI, but many like a check midway, some check daily or even several times a day.
I know that if I stick to plan, I should have a loss, having said that, our bodies do strange things, and sometimes it doesn't show until the week after - I know that if I've been on plan, and didn't see a loss midweek I'd feel disheartened, and if I did see a loss, I'd think I don't need to be so 'careful'
We're all different