Not bashing him in particular, but some of the dumbest things I've heard about guns came from gun store employees and owners. The Lasermax was DESIGNED specifically to serve that purpose, it's not like you are gluing a cat toy into your pistol.
You did not specify what TYPE of pistol, so I will point out that AS DESIGNED for the 1911 all the guide rod does is keep the spring straight and that can be done with a short stick, it takes no stress strain or force.
Using your dealers reasoning, extended guide rods, beavertails, and ambi controls are all verboten, as that's not how John Browning designed it.
Just use your own judgment, if you had doubts enough to ask for a second opinion here then your not going to go to far wrong.