Pennies

So why do people choose wide apertures?

I would have thought to use the smallest aperture I can, to maximize the depth of field. My digital camera has a macro setting, which let's me set it on a table-top tripod just 6 inches from the coin, but that gives a very shallow focal plane, to the point that if the coin is not exactly parallel to the ccd chip, some of it is not in focus. So just using a small aperture, longer shutter speed fixes that.

Also, from my 35 mm days, most lens distortions and imperfections are at the outer edges of the lens. So I tend not to use the outer edges, by stopping down a little bit. Granted, this may not be a problem with the smaller lenses used for digitals, since smaller lenses are easier to make.

Wow semi dead thread but.
Aperature
well all the way to f 8 isn't a must but wide open is likely to not carry the focus if you are at an angle and if you camera is off a bit wide open will never be there. In the dark room we use to close the aperature down 2 notches it was well known that most lenses focus the best a couple clicks from wide open. with todays computer designed lenses I don't know if thats still true, but would assume so.

shutter speeds; even with a tripod if you have a camera that flips it's mirror before exposing ie any SLR you will get camera shake. find out if you can lock the mirror up before triggering the shutter and use a remote shutter release.
I am finding that lighting is very interesting with coins. If you have a mirror like proof you want to show the wonderfully smooth reflection but at the same time you need to show the depth of the die strike. I recently was shooting a Morgan silver dollar that refused to co operate. I would look at it and it was mirror like (not proof like but a good shine) with nice deep details. I would shoot it and the details looked flat. so I went for the depth of the strike and the coin looked dark. it was a bugger. I guess I'll try it again later with the beam spliter (axial) light