If you shoot JPEG, not necessarily, since some basic PP is done directly in camera, basing on some standardized operation.
If you shoot RAW, you always have to do at least the part of PP corresponding to traditional film development. Someone here on the forum compared JPEG to bringing film to a lab, RAW to developing by themselves: more freedom but also more work.
At time of development, it becomes usual to help the picture be like you wanted to (e.g., if it is leaning, why not straightening it?). All things always done also by traditional photographers in dark room. Of course, the better is taken since the beginning, the better will be the final product.
As I shot RAW, 100% of my pictures needs PP (through free Canon software or, recently, Lightroom). Then, some needs straightening, many needs slight recomposition but only few some major recomposition (typically when I try to recover something from a picture badly shot since the beginning), a fair number need some work on dark areas. I also play with skin softening/eye sharpening in wife portraits (if not, she will no more act as a model for me

).