First off, no way you *need an HD wire for a 6" extension, especially on a 70" plane. Don't get me wrong, I like heavy duty stuff, but it is totally not needed. I have used plenty of HD servo extensions. If the price is right, length is right, and I'm not trying to save every gram of weight, sure I would use them. But the question is about need. Most of us don't need HD servo extensions.
Wire acts as a resistor. The longer the wire, the more resistance it has. Heavier gage wires have less resistance for the same amount of length. There are fairly simple calculations to determine how much loss in signal power you will lose for a given length of wire. You need to know how much current the servos are drawing. I don't know the answer to that, but I know that the BEC's or regulators I use deliver up to 5A continuous and I don't seem to max them out, so 5A seems like a safe number to use.
I just found this website, they have a chart for wire length vs signal loss for different sized wires:
http://www.securityideas.com/howtocalvold.html
I am building a 78" high performance airplane. I chose regular servo extensions for everything except the elevators - the elevators are getting a 42" extension. I am running one 42" extension plus an additional standard gage signal wire, so the HD power wire is carrying power for two servos. I have 6" plus 3" extensions for the ailerons, none are HD. Servos are probably rated around 150oz-in.