As someone who worked at HD, it entirely depends on the mentality of the person that does the cuts for you. If you get the older guy that loves helping people then you'll probably be good. If you get the department head that has to help you himself because everyone else called out sick? You better ask for an inch bigger than your actual final dimensions.

Mid morning on a weekday is the best time to get the former. That's when the retirees work and it's after the early contractor rush.