Do shower doors for tiled openings have to fit the width exactly?
Almost any such door has a range of adjustment or applicability, often in the form of panel lap. Tiled openings just aren't that precise. Look at the installation manuals for any models you like. They'll tell you what the range is.

For example, this one specifically lists 56-60". 


  Up to 4 in. of adjustment for width, if needed