Do I need a screen saver on an iMac late 2012?

Mac OS has a screen saver built in. System Preferences - Desktop & Screen Saver.

Is that what you mean or are you referring to some type of hardware to put over the display? In my 15 + years of owning iMacs I have never had a need for anything like that. However if you have little kids with bad tempers or you have a bad temper and throw things then well yeah maybe it would be a good idea. :-)

I never do, what I do is set System Preferences - Energy Saver - Turn Display Off After X and you set the amount of minutes. I have been doing this for years and have never had display issues.

No, in fact it's better with no Screen Saver running & just blank th Display as rkaufmann87 says.