in 6.1 the additional speaker goes behind the listening position, usually on the back wall, central to the left and right surrounds.
Most DVDs only have DTS or DD encoding which is 5.1 so your AMP will probably 'matrix' or 'fiddle-about-a-bit-and-add-a-new-mix-to-the-rear-centre' to improve the soundfield.
Some DVDs have DDEX which contains a pretend extra rear channel and only DTS ES Discrete has a real channel dedicated to the centre surround. (someone confirm this please, too many formats to remember )
Personally I use 5.1 because a) my room really doesn't need it (too small) and b) my surrounds are awsome. It is only my opinion but unless you have a larger room you may not benefit all that much from 6.1 or 7.1