Seagoll is half right- the company can't produce a flight plan/pairing which has you into discretion even if you push on time, which explains some of the wildly optimistic taxi times you sometimes see. However, dispatch has nothing to do with it. You can use discretion any time after you have signed in if, for example, an aircraft goes tech before the passengers have borarded and you have to do an aircraft swap. That is one of the contingencies discretion is desgined to cover. I don't mean to be pedantic about this but if we are going to argue with crewing about it (and if we say no be ready for a healthy discussion!) we need to know the rules inside out.
Halas raises an interesting point though- you shouldn't need to go into discression out of Dubai as there should be enough standby coverage.