My opinion is that this time is owned by the crew since everybody who has ever flown comercially know that blocking off exactly according to schedule or CTOT is not easy.
It's not easy, but the -5,+10, while definitely "owned" by ATC, does give you a fairly flexible window. While an ATCO is perfectly justified in holding you on stand if you won't be airbourne by +10, obviously he/she's not if you would be. This +10 is of course so often the difference between staying or going. However, if you miss your slot, tough luck. Them's the rules.