Most were photographed outdoors on the back deck in good light, shaded from the sun. Don't ever put original artwork in the sun!
I've photographed some artwork in indirect light (actually on the back deck!) as well. With non-directional light, you get less of a sense of the texture of the surface, at least in my opinion. It may simply be question of what you're trying to show.
I also find that direct sunlight (or intense direct light from strobes) picks up colors slightly differently than indirect light on some works. I think it may be because many paints, even those that are thought of as opaque, have some degree of transparency and so allow the viewer a little sense of what is behind them. Intense light may penetrate the paint more deeply and reflect off the surface beneath the paint while less intense light may simply reflect off the outermost layer of paint. I should note that my experience is mainly with watercolor, acrylics, pen-and-ink, collage, and prints. I can well imagine that other media, like oils or pastels, might behave differently.
While I certainly would never display a painting in direct sunlight for any extended length of time, I've never had any concern about exposing modern paintings to sunlight for the couple minutes it takes to set up a shot. I suppose old masters or some particularly fragile types of media might be a concern.
As an aside, I have from time to time seen multiple reproductions of the same work by a famous artist displayed side by side. Even when the reproductions are all high quality, professional, reproductions marketed for a critical audience, it is surprising how much variation one sees in the different "interpretations" of the same work.