In general, whether or not you should go with mounting brackets depends on the weight of the speaker, the direction that the port on the speaker fires out, and the room configuration. In general, the diagram below shows the most optimal arrangement for a surround speaker setup. Dolby also recommends that you raise the speakers by about two feet above ear level and point them directly at one another.

Generally, wall brackets are not designed to support heavy or large speakers. Also, if your speakers have rear firing ports, then you don't want to mount them too close to the walls, which makes wall brackets less than desirable. And if your room is really wide, you might want to look into tall speaker stands instead.

Another thing to look out for is with the center speaker. If you plan to use a built in entertainment unit, the center speaker generally should not be installed inside an enclosure. Inside an enclosure, the center speaker will reverberate more than in free space, and you'll lose some focus with the center imaging.