>>>do I need a balun for a G5RV? <<<<
You should construct a simple coax coil 1:1 balun at the connection point to keep RF from being reflected back down the outside of the coax.
The antenna's inventor:
"Louis Varney warned against the use of transformer-wound 1:1 baluns because many designs show considerable losses when the load reactance is significant. Indeed, Jerry Sevick, W2FMI, who has published the most material on transmission-line transformers, recommends that all reactance compensation occur on the load side of the balun.
In place of such baluns to suppress common-mode currents, Varney recommends a 6" diameter coil of about 8 to 10 turns of the feedline coaxial cable at the junction of the matching section and the main feedline. "
I have used that ~6" dia. coil with success. Wireties, or electrical tape to hold it in form and in place. Nice and neat, not scatter wrapped.
Beware of those around the web claiming that you should not use the choke balun, hams can be fooled by better performance locally by that radiating coax.
Check out this article: http://www.cebik.com/wire/g5rv2.html