Do we know what type of battery is inside the Mojo? If not, are we assume it's a Li-ion?
If we are assuming it
is a Li-ion then yes, there
is benefit to leaving it charged past when the light goes out. It is well established that Li-ion batteries should be left to charge for extra time to be sure the battery is truly charged to its full capacity.
Again, this
only applies for the very first charge you ever make on the device, and the moment you turn the power button "on", is there the battery's "memory" begins and any possible capacity remaining is effectively walled off. This does not necessarily apply to non Lithium Ion batteries.
If the Mojo does not use Li-ion batteries then you may disregard what I'm posting here, however, the fact that the designers of the Mojo say on the box "minimum 10 hours", it sounds to me like they're using Li-ion batteries and instructing people to do the first charge for the proper amount of time without getting overly technical, which is exactly what I'm doing right now.
So if you didn't wait 10+ hours before turning it on, or you used it while doing the initial charge, yes your Mojo still works fine and will continue to, but if you get 8 hours of use, maybe you would have gotten 9 if you had charged it 10+ hours before turning it on as instructed.
If the possibility of squeezing that extra little bit out of the battery isn't worth not being able use your Mojo within 10-12 hours of getting it, then by all means go ahead and use your Mojo soon as you get it. I personally would be bothered every day that maybe my battery wasn't as perfect as it could be so I'm doing the 10+ hours before powering up.
*deep breath*
Once again.... this is all just general discussion of proper battery conditioning techniques. Hopefully someone from Chord will pop in and say what kind of battery it uses for those of us who care. This is already way longer of a post than it needed to be