This article is from the comp.sys.ibm.pc.hardware.video Frequently Asked Questions, by Michael Scott with numerous contributions by others. (v1.0).
[From: Sam Goldwasser (sam@stdavids.picker.com)]
The quick answer is: it is not as important as it once was with fixed format text displays. I still recommend using a screen saver as cheap insurance if it uses mostly dark colors or black. In this case, your power dissipation and heating will be slightly reduced as well. However, even completely blanking the screen with the video signal does not significantly prolong the life of the CRT. (See the section on Monitor Life.)
The best solution is to use a 'Green', monitor in its full power down mode. This saves energy and wear and tear on the CRT and other monitor components. Most newer PC BIOS's now support energy saving modes in the CMOS setup. These should be used but only if your monitor specifies that it supports the relevant energy saving modes. Otherwise, you could damage the monitor.
 
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