Along the same vein as Dunkrige's post regarding where to open your chests is when to open chests. Odin's Bounty is the first event that I know of which allows you to easily stockpile boxes full of XP. They don't have to be opened during the event. In fact, it's probably better if you don't open them during this event.

During the Operation Omega event, server side XP boosts went to 192% on the final day, which is almost twice the 100% boost of the current event (which itself is not normal, and only set because of problems the event had in the beginning.) OO also has a 24% Omega Point gain increase, which works in conjunction with the 192% increase. Open Odin's Bounty chests on the final day of Operation Omega for the maximum benefit, especially if you're trying to farm Omega Points.

Assuming they fix Cosmic Chaos next time it rolls around, the 50% reduction in Cosmic Prestige levelling might make it a better time to open chests then for your gold heroes. Be sure to verify it's actually working before popping all the chests, and remember that Odin chests have a level requirement, so you can't stack up 3 stashes of level 60 chests and expect them to help your Cosmic Prestige characters level...

In short, just because you receive chests during this event, that doesn't mean this event is the best time to open them.