Bob, the first point is that in giving distances from room surfaces the relevant number is the distance from the center of the woofer cones(not the enclosure)to the surfaces. As Chris points out, it's desirable to have the distances from the three surfaces(walls and floor)unequal, so that reflections from them don't pile up at the same frequencies.
As long as the ports have at least a few inches of "breathing room", they can be placed as desired. Separation should be about as much as your distance from them, with enough toe-in so that the tweeters point at your ears.
