A tilt of two degrees is invisible while you take the photo and impossible to ignore afterwards.
Use a real reference rather than your eye alone: a horizon, a door frame, the edge of a table, the bottom of a page. Line that up and the rest of the picture follows.
Rotating always costs you a little of the edges. Turning a rectangle inside its own frame leaves empty triangles in the corners, so those have to be cropped away. A small correction costs almost nothing, a large one eats into the picture.
For a photographed document, straightening makes a real difference to how it reads and how seriously it is taken. A page at a visible angle looks like a snapshot rather than a scan.
A little around the edges, since rotating leaves empty corners that need cropping. Small corrections cost very little.
Rotate in small steps until a reference line looks true. Most crooked photos need only a degree or two.
No.