Ground location Vs. Sending unit location.

When I installed My Olde Tyme White Gauges, the Destructions said to Ground them close to the senders. I always thought that Good ground was good ground. So I called Autometer Technical Support. They told me that the gauges would read lower the farther away the Ground was from the sending unit. Is there some truth to that? and if so, how much affect does it have?
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