What is your mileage? If you are riding a few miles at a time, a couple times a week, then you have nothing to worry about. If you start actually being serious about road cycling, then you will have to worry about the correct adjustments - saddle height being
the most important.
It is true that a saddle which is too low can lead to some pretty bad knee problems. If you are riding lots and trying to push yourself, you should definitely listen to the bike shop employee. You should not be positioning your saddle according to how well you can touch the ground. The ground is irrelevant to pedaling. The rough rule of thumb is to place your heal on your pedal while you sit square in the saddle (have someone hold the bike for you, or lean against something. You should not have your other foot on the ground) and your saddle should be adjusted so your leg is locked in that position. Then, when you put the ball of your foot on the pedal, your leg is mostly straight, but with a little bend to it.
Anyway... Point is that if you ride with any sort of intensity and for any significant distance, then saddle position is important and you shouldn't
it away. If you are just spinning around the bike path a couple miles a week at walking pace, then you have nothing to worry about. Most of the cyclists in the "Road Cycling" forum are the former; they want to get seriously into cycling and so saddle position is extremely important to avoid repetitive stress injury.