Buzzkill and Derrin,
Thanks for the input.
My way of thinking (apparently incorrect, ha!) is that if the cold water entered at the top (where the hottest wort is) it would then, in effect, be heating up the rest of the wort as it progressed down the coil. If, on the other hand, it entered at the bottom, it would still have capacity to absorb heat from the hottest part of the wort as it progressed to the top to the exit.
This choice happily coincides with Derrins point about air pockets so that is the way "ll go.