Photo credits: Previous Page, top, Shop Pop Displays Previous Page, bottom, Vancouver Art Gallery This Page, top left, Sarah Wilson
You may choose to use frames that match one another, or you may not. If the pieces are of a similar theme or color scheme, it can be helpful to make the frames all match so that the pieces are the most noticeable change throughout the wall. Of course, if the pieces are very different, it may make the most sense to give them each their own visual identity with separate framing styles, but it is generally one or the other. Either it is all uniform, or none of it is. Galleries today often use black or white frames with white mats, but this is hardly a rule.
If you choose a uniform frame style, be sure that the frames support what the piece is really trying to accomplish.