A question often posed by beginning watch photographers is, "Should I use the flash"? I'll give the short answer as, "No".
Of course, there's also a longer answer. I have said "no" assuming that most people will be referring to the in-built flash unit on their camera (or a unit attached via a hotshoe). Flash lighting does actually have widespread use in professional still-life photography. However, the equipment used and the manner in which it is used is quite different to blatting away with the on-camera flash. Firstly the flash units used are separate units used off-camera, generally referred to as strobes. By moving the lighting away from the camera we minimise the chances of the lighting reflecting straight back into the lens and allow good modelling rather than the frontal "flat" lighting on-camera flash will provide. Apart from being off-camera, the strobe units are generally diffused, often via the use of large units called "fish fryers".
At left is an example of a fish fryer. These can be quite large and sometimes house multiple strobe units. A bare flash unit presents a small point source of light which can create specular highlights, especially when dealing with the many reflective surfaces found on watches. It is important to diffuse the output of any flash units to modify the nature of the light. Diffusing the flash units, particularly with relatively large diffusers, will turn them into large even sources of light. This not only provides a less harsh light, but also results in large even reflections rather than bright pinpoint highlights. Portrait photographers often use umbrella reflectors, wherein the strobe unit is directed into the umbrella which acts as a reflector. Again the aim is to present softer less directional lighting. Another important difference between strobe lighting units and the on-camera flash is that the strobes generally have integral modelling lamps. These are conventional photo lamps that offer the photographer a continuous light source and the ability to judge, at least roughly, how the flash lighting is going to look. With the on-camera flash we do not have this, and there is no way to judge just how the lighting will look until the exposure has been made. Thus, it is largely hit & miss until some experience has been gained and settings decided. With subjects such as watches there will always be multiple reflective surfaces to deal with and without knowing beforehand how the results of the lighting will look there is always the likelihood of unwanted reflections.
Below are two images that illustrate the results gained from, on the left, undiffused on-camera flash, and on the right a diffused continuous lighting source (in this case a fluorescent lamp). The flash-lit image shows the harsh nature, specular highlights and high contrast common to such lighting.
So, to get back to the short answer, turn the on-camera flash off, unless you are willing to use off-camera flash which has been diffused. With the flash off you'll need some form of continuous lighting, be it natural (sunlight) or artificial (incandescent or fluorescent). Such lighting can be quite inexpensive (sunlight's free!). You will need to ensure the camera is white-balanced to suit the type of lighting. Info on white-balance/colour temperature can be found HERE.
I wouldn't want watch photographers to completely dismiss flash lighting, however, but rather to understand the issues involved and that it is probably going to be easier to obtain good results in a home studio by using simple continuous lighting. An area where flash lighting is particularly handy is when shooting away from the home studio, perhaps at a watch show or anywhere it is impractical to set up even basic continuous lighting. Flash lighting provides a handy, self-contained, mobile lighting source. Keep the issues in mind - move the flash off-camera and diffuse it. I have seen very good results from people using a handheld flash unit diffused through a small light tent. A benefit of flash lighting is that it is of short duration and, given low ambient lighting, it can do away with the need for a tripod. Below is an example of an off-camera flash setup I used back in my film days for close-up shooting.
As you can see I used two small flash units mounted on brackets attached to the camera to provide lighting from both sides and overhead. The flash units have been fitted with diffuser hoods, both to cut down the intensity and to provide a softer light. Thus I had a portable, self-contained, lighting/camera unit that I could move readily around from subject to subject, providing a consistent lighting. Although shown mounted on a tripod here I actually used the unit handheld. I was shooting mainly nature subjects such as the flower and insect example images shown below, in locations that did not practically allow any other type of lighting. Reflections were not a major issue with such subjects and for shooting watches I would want the diffusers to be much larger. I have not used such a setup with watches and show it here only as an example of the use of off-camera flash lighting.
Copyright 2006 Paul Delury