Putting glide wax on the grip section of a waxless ski is a good way to make a mess in the very best case, and might ruin the ski in the worse case.
There are various goops and jellies (MaxiGlide (tm) is a common brand, as is Swix F4) that are meant to be applied to the entire base of a waxless ski. They reduce friction a bit but help most in keeping the ski base from icing up. I hope that was what your salesman was talking about.
As for actual glide wax applied to the tips and tail, you ought to do it at least twice a season (once at the start and once at the end) and leave a lot of wax on the base over the summer. That will greatly increase the lifespan of your ski and make it faster to boot. Skiing on a slow ski is like poking your eye out with a dull stick.
Here's my executive summary of how to apply glide wax to a waxless ski.
What you need: an iron (from goodwill), some glide wax (from a ski shop), a plastic scraper (also from the ski shop) and a plastic brush (ditto).
Step 1: turn the iron on. The iron should be hot enough to melt the wax, but not so hot that the wax smokes. A too-hot iron is a good way to wreck your skis.
Step 2. Take a bar of the glide wax and rub it on the base of the iron to melt it a little, then use the bar of glide wax like a crayon on the tip and tail sections of your ski base (everywhere but where there is a grip pattern).
Step 3. After you have crayoned plenty of wax onto the ski, apply the iron to the parts of the ski with wax on them. KEEP THE IRON MOVING. Move the iron slowly from tip to tail, avoiding the grip area.
Max three two five passes on each ski.
Step 4. Set the skis aside for about fifteen minutes. Have a cup of coffee.
Step 5. Get the plastic scraper and scrape all of the excess wax of the ski base. You've scraped enough wax off when you can only scratch a teeny bit of wax off with a fingernail. Move the scraper from tip to tail only.
Step 6. Brush the base thoroughly. Once again the brush should only move from tip to tail.
This may seem like an extremely bizarre ritual. But it does produce faster skis. Nobody really knows why. Myself, I think it is related to The Great Ketchup Mystery.