Yup ... I've been doing it for years - especially with photos taken in the UK where skies are often dull and featureless. I don't use Photoshop, I'm a Paintshop Pro person, but the techniques would be similar.

It has become a matter of "have to" given that competition is so intense these days. There are fewer photos straight out of camera and if you enter comps you're going to be up against lots of manipulated images.

Always easier and better to do it straight out of camera, but if you are only in a place for one moment in time, and that moment has crap skies, then you do what you have to do.