generally speaking, the maximum voltage for laser pointer pens is the voltage of two alkalines, can be up to 3.2 or 3.3V.
The idea behind the diode is to lessen the risk. There is still risk, and again you must decide if the risk is worth it to you. The benefit, though, is being able to supply MORE voltage, because more voltage basically means that the battery can push the current through the device more strongly, which is why you would want to do this modification in the first place.
I have a cheapo amazon green laser pointer that I use this mod for. The pointer cost me $12, not to mention it was a gift card anyways, so I felt like if I fried it, I don't really lose that much.
I also have a more expensive laser pointer that I will NOT use a 10440 with, because it is the nicest completed product that I own. I do, however, use AAA NiMH Duraloops with it.