I guess this "LiCo cells should be charged to 4.20 Volts" myth will never go away. This discussion comes up over and over again.
It's really fairly simple. As far as I know, there is no specified voltage for which any Li-Ion cell should reside at, when fully charged and taken off the charger. The only specification is the maximum voltage of the charging circuit during the CV stage of the charge. For most LiCo and LiMn chemistry Li-Ion cells, this is 4.20 Volts +/- 1%, or 4.20 Volts +/- 0.05 Volt.
When a cell is removed from the charger, the cell voltage will decline, to some degree, depending on the age of the cell, it's use history, actual ingredients and compoents used in the cell (which varies from manufacturer to manufacturer), and so on.
I think part of the problem may be confusion between the terms "charging voltage" and "charged cell voltage". These are two different terms and are not really directly related to each other. As a result of the CC/CV charging process however, a cell will be limited to a voltage which is equal to, or less than the CV voltage of the charger. It will never be higher.
It alarms me that many strive to charge their LiCo, or LiMn cells so that, after charging, they read 4.20 Volts off the charger. Some have even gone so far as to modify chargers (usually inexpensive ones that don't use a proper charging algorithm to start with) so a voltage reading after removal from the charger of 4.20 Volts can be obtained.
This modification is potentially damaging to cells, and IMO, a dangerous practice. The older, or more used a Li-Ion cell is, the more likely it is to become unstable. The worst thing you can do to an old, or unstable cell, is to charge it to a higher voltage than is acceptable, for that particular cell. In addition to causing unnecessary wear to the already worn cell, there is some potential that the cell may "vent with flame" under these conditions. The CC/CV charging algorithm is used, not only to extend cell longevity, but as a safety measure, as well.
So, one more time, "charging voltage" and "charged cell voltage", as pertains to all chemistry of Li-Ion cells, are two different voltages, and should not be confused. The first is the voltage of the charging circuit (cell included) and the other is the voltage of the cell independent of, or removed from the charger. These two voltages will very rarely ever appear to be the same. I say "appear to be the same" because the cell voltage independent of the charging circuit, will always be less than the charging voltage. The only reason the two voltages may appear to be the same, is the limited resolution of the voltage measuring device, and this situation will likely only arise with very new cells.
Dave