No problem for ICAO if you choose a lower level (if not limited by other factors); any company has his own SOP, but normally no problem.
Usually, planning for FL370 and then flying at FL240 would need too much more fuel, so probably you also need a replanning to stay inside the minimum block, or at least have reserve fuel intact upon landing.
By the way, going back to the question, the cost difference in fuel burned vs flight time is different for each plane and for each company (you can see that any company has a different cost index), that means that there isn't an absolut answer to the question....but if it was up to me...at least with the oil price of few months ago...i'd go for high level, no question.
FB