I don't remember where I heard it but "It is OK to disappoint but not OK to surprise" rings so true. Also, the earlier you know you'll be late the bigger the chance is that additional help can avoid the late delivery.
I always try to front-load the riskiest and most difficult work on projects for this reason: that way you're never in the situation where everything looks fine until it all goes off the rails at the last minute.
I've never heard that saying btw but it's a great one, I'll definitely remember it for future use!