Unless the rears are more than 80' from the amplifier, I wouldn't bother and using the same gauge for front and back won't be audible. The rear speakers don't receive as much signal as the main ones and because our ears are angled forward, a lot of the detail isn't noticeable when it comes from behind.
I have known someone who has worked as a sound engineer in recording studios and live sound, as well as doing audio system installations for over 35 years and when larger speaker wire first became a topic of discussion (in the early-'80s), I asked him about it. He said he had done TEF analysis and found that 16 ga is fine for live sound, up to about 60'. Since 'live sound' systems for large spaces or loud music usually operate in the HUNDREDS of Watts per speaker for a large part of the show, this is an important point. When a speaker system is only receiving a few Watts, heavy gauge wire just isn't necessary. It might look cool, but that goes away as soon as the lights are turned down, someone closes their eyes or leaves the room.