I just researched the same thing as diodes would have been the easier install for me. But I decided to go with separate bulbs.
For the simple reason that the toad has absolutely no common power from the towing vehicle. With these new cars that have many functions controlled by the ECM and other computers, even a small signal back into it from a failed diode or whatever could be a cause of error codes or trans shifting out of N on an automatic. It would certainly be an excuse to deny your warranty claim.
Although I am 99% sure the diodes method would not be a problem, I went with separate bulbs and connected the ground only to the bulb sockets and not the chassis.