ORIGINAL: Sandmann_AU

Technically wire's a low value resistor, and no solder joint's a perfect conductor so yes, making your servo leads longer will reduce your battery life, however the resistance values are so low that it's not going to make any difference to your flying time. You'd need some majorly sensitive equipment to even measure the difference in resistance. As Brett65 pointed out regular servo extensions have a higher resistance in the plug & socket.

An interesting point re soldering & vibration is that if your joint's got air bubbles in it and you get the vibration is just right it can actually turn the joint into a capacitor which will block DC current. They started finding this in early (full scale) jets when they started getting near the speed of sound and the on board electronics would suddenly start shutting down. It's one of the reasons we're now using wave soldering techniques in industry instead of traditional hand soldering (other than the obvious cost & speed benefits). This isn't something that's likely to bother us though.