Your company has to give guidance as to when the MEL becomes operative. I've flown several different types for my employer and each had a different policy. My current type states that the MEL applies until "Off Blocks". Previous types have defined the start of the Take-off roll. Another defined the point as when taxiing commenced. But whether you go or not still rests with the crew and if you are at a maintenance base with spares and engineering support, a departure even though legal, may not be the most sensible thing to do.
PM