aint no expert, but i find what some ppl say to be true, as i myself do that most of the time.. some ppl say that the subject in a landscape/scenery shot should be in the right corner lower 1/3 as the eyes scan from left to right , top to bottom... (thats how i view pictures, documents etc..).. so as the eyes scan the pic, they will end off at the bottm right corner.. hence, some would say that the subject should be placed there for emphasis.. think it works for top left corner as well ..
whereas, for macro shots and maybe some form of full body portrait shots (landscape layout), where you would want to isolate the subject from the background, putting the subject at any place other than central/slightly off central seems wierd to me.. the background would be blurred out for isolation, and will seem like wasted space (to me , at least)
my 2 cents..