In the piano world, the view is that it is the ends of the wood fibers in the
wrestplank that hold the tuning pins in place. It is the flexibility of those
fibers that allows the pins to turn during tuning without enlarging the holes
in the wrestplank. The sides of the fibers do not contribute to holding the
pins at all. This is one of the reasons that wrestplanks in pianos are made
with multiple laminations, with grain oriented in three directions in some
cases. Logic would suggest, therefore, that whether the planks were flat or
quarter sawn would make no difference in the way the tuning pins turn.
One the other hand, piano makers go to great extremes to secure the wrestplank
in the piano (mortising and doweling it to the case on three sides and shaping
it to insure an absolutely flush fit to the cast iron plate on two other
sides), in order to insure that the plank does not move around, causing tuning
instability. This suggests to me that strength and stability are an issue and
that a quarter sawn wrestplank would be an advantage.
I will be the first to admit that my knowledge of the construction of
harpsichords is limited, and I would be very interested in hearing the
opinions of others.