That’s not quite true Ray as the default is auto which means the absolutely placed element will be positioned at the point it was in the flow.
Most of the time that will appear to be the left side but if the parent had text align center or text align right then the absolute element would be central or to the right without positioning assuming it was an inline element before being absolutely positioned.
It was common practice to position drop down menus using left auto when only the main parent (the ul usually) had position relative yet the drop menus would always line up with the top menu items as required. Old IE used to get this wrong so everyone started using left:0 and adding position relative to the list item instead.
Therefore unless you specifically want auto positioning behavior it is better to always supply at least two co-ordinates for your absolute elements just to be sure. 