As a general rule, grouping similar objects together in groups of three works better than a pair of things. Eclectic collections supply visual interest and random picture frames look perfectly find as long as there is variety to create an engaging visual feast. When displaying only a few objects, make sure they are in keeping with the overall style of a room. Three church candles on a mantelpiece will look good in a simply styled room, for instance, but will get lost in a room where riotous colours dominate.
A lined-up display works best if all the individual elements are quite different from one another. In Vicente Wolf's New York loft he has ranged a collection of black and white photographs along two seamless shelves to create a gallery beneath which many styles of occasional chair make a witty parade.