Proper use of "smooth image"

I cant decide if I will smooth an image or not, because sometimes there are no visual difference. And sometimes when I unsmooth an image it becomes distorted.

So, are there any standards or proper use of this feature?
For example, do I need to unsmooth all the images, to ensure that the game will be visually pleasant in different devices? Is unsmoothing all the images, will decrease the game performance? Or are there only paticular images that were needed to unsmooth (like pixel art)?

Thank you in advance :grin: