They use lithium ion batteries which have no memory characteristics like the oooold NiCd technology. You can use them as they are and charge when needed. They are usually shipped with a storage charge. No worries about needing an initial long charge.
The only thing I would say is that if you intend to store your battery for long periods of time without using, don't store them fully charged or fully discharged. That can damage them. Other than that, charge them when the voltage cutoff built into the eGo shuts them off or anytime before that. You can also take them off the charger before they are done with no harm other than you won't get as many hours of use before it needs charging again, but that should be rather obvious.