It would be a minor power savings to turn it off. I doubt it lowers the life span having it turn off. It would probably extend the life of the back light.

I think it would only be a problem if it was turning on/off several times a minute. Just a guess though, but probably still not very likely. I have only heard of screens breaking from physical damage (being dropped, falling off roofs of cars etc).

To add to what kffitzgerald said, laptops tend to block up with dust, really the only way to fix it is to disassemble the laptop and remove the fan to clean (app. once a year). However, you can't easily disassemble many laptops now. Another option is to used compressed air but this doesn't seem to work as well as disassembly.

The ideal would be to turn the screen off and on only a few times a day. I doubt that doing this one or two times more a day than you already are would make any noticeable difference,

Lawrence