Placing a piston, dropper, observer etc. facing down or up in any negative coordinates is very difficult if you're unable to stand directly over or under the position you wish to place them in. i.e. in -x and -z placing a piston in a 1 high gap to be facing either up or down can only be done while standing on the south or east side of the block where as in the +x and +z the component can be placed from any side of the block. In the attached image all the dispensers were placed from below onto the iron block above, the examples shown are centred on 0,0.