I wouldn't install the extra relay unless you just want to eliminate the power at the starter when you aren't cranking the engine, if you are planning to install one of the permanent magnet gear reduction starters. There is a small downside to using a relay with one if them. The starter gets a little bit of "run-on" and doesn't quit cranking as soon as you let off of the key. It doesn't bother anything since the overrun clutch is in the drive, but it sounds like you don't let off of start position at the right time. We did the relay in my '72 C10 with one of those starters and it does this. I have learned to let off of the key sooner in anticipation of the engine starting.