I just export the entire site and then decide what needs to be uploaded.
I'll usually try to take note of the things that are changing. If it's just some text and no formatting changes I'll just upload the new html files. If it's any style changes I'll upload html and css files. And if it's any type of imported files I'll upload html, css and the associated file such as a jpg.
If I've just made a change to a specific image, let's say adjusted a logo but all filenames are the same and no style changes were made I will usually upload the new image file to my host and also update my RSD document. But in that case I usually won't do an export.
If I've made site wide changes and changed lots of images then I just re-upload the entire site.
For me it all depends on the specific situation and changes.