I used to spend a lot of time making finesfull, photorealistic framing prevues in PS, complete with different lighting on the top and bottom pieces of moulding, drop shadows, you name it. There was only one problem. Images like that lie. They simply don't reliably predict what your actual piece will look like in an actual installation under actual lighting. The reality is always a surprise, no matter how "close" your approximation is.
Among other things, it is exceedingly difficult to get the visual weight of all but the plainest moulding correct, and there is a tendency to err on the side of what's pretty, rather than what's accurate. Where one gets in the most trouble is by taking pictures of moulding obtained online, and using those in previews. Online moulding pictures are NEVER accurate! But if shown to a client such images can commit you to providing the unobtainable.
There are some uses of even inaccurate previews. One is to evaluate the worthiness of a piece for presentation, albeit in the abstract. Another is to approximate how a framed piece might look shown against a variety of colored walls, and it's surprising how poorly the newly popular white frames fare in that respect. But be careful, don't mistake a pretty prevue for a meritorious image, the way Apple mistakes its stylish keyboard for something useful.