Sorry for posting into this old tread, but this really belongs exactly here.
Per my own measurements these laser pointer modules start lasing at full power if you feed them 2.5 Volts or more. Below 2,5V they start becoming weaker until they stop lasing at about 2V. If you supply a voltage over 2,5 Volts, the laser will not become stronger due to the regulation in the module.
3,6 V, or 4,2V will not lead to a higher current at the diode nor to a higher current at the battery, so the diode´s lifetime will not be reduced. Because of this there is no need for "turning the pot down". The lithium cell powered green pointers I own are all equipped with the same kind of module, the same kind of module which is used in the 2-alkaline-celled pointers.
But: The higher voltage will create significantly more heat in the module (not in the LD directly. Significantly here means about 100-200% more. This could in fact decrease the module´s lifetime, and it for sure reduces the overall efficiency.
Beside of this the laser could easily discharge the unprotected lithium cell down to 2,5V before you realize it´s empty, even a discharge down to 2V is possible, which could shorten the lithium cells lifetime with a high probability, if you don´t use a diode in series.
So the best idea is to use 2x1.2V NiMH. Even if I like 1-celled 3,6V pointers more, its only because of the form factor. Electrically two NiMH is the best choice.