Depends on where you live. I live in Albuquerque where the air is extremely arid. Evaporative coolers work great except during our monsoon season in July when the humidity increases from single digits to well above 30 percent.
The coolers don't work well when the humidity is high.
We have a couple of large flat screen TVs and a stereo system and it hasn't seemed to bother them at all.
Best thing to do would be to ask at some electronic stores in your local area where they are more familiar with your local climate.