srnet:
Fair point, although the original post has now been edited.However, a cut of voltage of 2.5V is way too low. Not a good idea to let the batteries discharge that low on a regular basis.
That depends on the battery. Usual 18650s has a discharge cut-off voltage at 2.75 volts but since the OP said he might use them Panasonic 3400s, which has a discharge cut-off at 2.5v.
Anyways these cheap modules are only providing this feature as a last line of defense for cheap unprotected cells. Good 18650s have their own built-in protection chip anyway.
Regularly discharging cheap cells to 2.5 volts will destroy them like you said but the OPs project will not use that much power to completely drain a 18650, unless he is planning to use an Ultrafire 10,000mAH crap.