No, a balun or choke should NOT be used on a G5RV wire dipole. Doing so will ruin the multi-band capabilities of this antenna.
The classic G5RV 102 ft. wire dipole consists of 51 ft. legs with a center feed of twin lead or ladder line that is about 34 ft. long. That section is also part of the radiating antenna, and is normally oriented vertically, perpendicular to the horizontal dipole wires and must not be on the ground. The G5RV is intended to be installed with a direct connection from the bottom end of the ladder line or twin lead to at least 70 ft. of coaxial cable. That length of coax is specified because it is part of the tuning system to make a G5RV dipole offer low (not perfect) SWR over many bands, 80 through 10 meters, as intended. This is why the originator recommended against using a choke or Balun at the transition between the ladder line and the coax.
The only place a 50 ohm feedline choke may be used is at the station end of the coaxial cable, which should be be at least 70 ft.. In this case, for the G5RV antenna system, a feedline choke cannot be installed at the antenna end of the coax, and is especially useful at the station end if the coax is not lying on or in the ground.
Some G5RV antenna manufacturer instructions say to use a 10-turn coax coil choke or Balun at the antenna feed point. Do not. That would make it fail to function on several bands. Others recommend a choke at the transition between the ladder line or twin lead and the coax. Again, choking the antenna end of the coax feed line will prevent the G5RV antenna from providing intended multi-band tunable operation.
The G5RV will require the use of a wide range antenna tuner in the shack. Often, the tuner built into the transceiver cannot handle he task.
Every antenna installation may have unusual circumstances that cause unintended or unexpected operation. However, a G5RV, if installed high and in the clear, away from interfering metal, it should not cause RF feedback. We recommend that the G5RV be positioned at least 45-60 ft. above ground for the proper tuning and operation.