I can answer your question 100%. That is, if you are bi-wiring one set of your RF7 MkIII, you can leave the impedance switch in "High", just remember to pull the jumpers.
The two sets of terminals are there for convenience. When you select A+B, it simply connect the A and B binding posts together, that is, red to red and black to black.
If you are using two sets of RF7 MkIII one connected to A, and the other to B, then when A+B are selected, you will have to set the switch to "Low" because the combined impedance of two sets of the 8 Ohm RF7 is 4 Ohms.
However, as Gene responded, if you set it to Low, you will be effectively limiting the power output of the amp, so it is up to you. For me, I would find out what my actual power need is, and then make an informed decision. Yamaha, however would have no choice, but to tell you to switch it to "Low" for regulatory and/or liability reasons.
Again, for bi-wiring, with just one set of speakers per channel, there is no need to worry, leave the switch in the default 8 Ohm position. I hope that's clear.