If you are worried about what does or doesn't void a particular warranty, you should visit the company's Web site and read their terms.

Given the fact that wall mounting of flat panels is incredibly common, it borders on the absurd for a repair person to suggest that it automatically voids warranties. 

Here is an excerpt from the Panasonic 50" G10 Canadian warranty:

LIMITATIONS AND EXCLUSIONS
This warranty does not apply to products purchased outside Canada or to any product which has been improperly
installed, subjected to usage for which the product was not designed, misused or abused, damaged during shipping,
or which has been altered or repaired in any way that affects the reliability or detracts from the performance, nor
does it cover any product which is used commercially.