I've tried it both ways. What I experienced was a higher internal resistance and quicker capacity loss with charging packs ahead of time compared to just in time. Same as most others who test have discovered.

Doesn't mean it can't be done, I just think it's worth knowing the tradeoffs. Batteries are disposable anyway and can be had fairly inexpensively, convenience is probably worth the reduction in performance for many, that's fine.

One item to be especially careful of is temperature changes while charged. Charge state changes with temperature, don't leave a charged pack in a parked car sitting in the sun for example.