Will the quality of my JPEG images taken by my iPhone deteriorate if I rotate than on my Mac?
Using Preview.app, the rotation applied is not lossless. You need to use a special tool that will either add an EXIF rotation header (which is then applied only on display) or simply keeps the encoding coefficients constant, but shifts them accordingly. One tool that supports this that has a UI is Xee, on the commandline you could use jpegtran or imagemagick.