Aaron wrote:
Hi,

A plumber told me that it's a good idea to shut off water from the
washer when it's not in use because the flexible lines aren't rated to
continuous pressure.

A. Is that true?
B. Does the same apply to the gas line for the dryer?

Many thanks in advance!

Aaron

Yes, it is a good idea. The metal braided lines will last longer than
the rubber hoses. I installed a one-lever shut off valve, about $20.
The gas line should have a shut off value nearby, but it is not
necessary to close off this (low-pressure) valve.