Oh man I do this exact thing (pre-checking) on a couple of my goals, and I can’t decide if it’s A Problem 
I don’t think it’s really a Problem for my toothbrushing goal (I check it off before I go upstairs for bed, because as Beeminder support can testify, I never ever remember to check it off once I’m upstairs!) because if I pre-check but don’t actually brush my teeth, I make sure I derail the next day. It’s also a pretty low priority goal 
But for my “bedtime routine” goal, the actual time of the deadline is important, and I keep skating the edge by checking it off and THEN doing my routine. I started doing it when my daughter started taking hours to go to sleep, so I legit didn’t have time to do the whole routine by 9:30pm, but the habit has stuck even on nights when she falls asleep quickly, and even after I shifted the deadline to 10pm 
I’m thinking maybe I should just lower the slope to something where I can meet the original deadline even with the more-than-occasional “bad night”.