If you see a photo that has been straightened you likely won't notice it. Not unless you see the original. The most common use of straightened photos is for architecture where you want to see things like they were once drawn.
Straightening a photo is relatively easy nowadays. You can do it on your phone and there are apps that do it for free. I like Snapseed from Google. It has one of the easiest to use straightening tools I know. Just open the photo, open Tools and choose Perspective. Then you just drag the corners until the subject is aligned with the horizontal and vertical help lines.
(Photo by Jordan Bishop)