Many (if not most) customers will have personal preferences regarding paper finish, so be sure to ask before you print, as printing is expensive regardless whether you print at home or send it out. You don't want to have to do it twice.
If they don't know how to choose, you could give them some guidelines. The type of paper I use personally greatly depends on the viewing situation. If the document is going under glass to be displayed vertically in a living room, where the lighting is relatively random, I use a matte finish to cut down on awkward reflections. For display at a show, under controlled lighting, a glossy finish really yields the full contrast gamut. For photo albums I like semi-gloss to bring out the blacks but not show every fingerprint.
I also use a glossy finish for photos that are to be reproduced, such as in a paste-up layout, although since layout is almost universally done on a computer these days it is less useful.