Everyone has a different way of forming the G Chord, that's how he prefers to play it. The reason is to do with switching speed and maintaining rhythm when changing chords during a song. Play the G and all other chords in a way that's proper, but comfortable for you.

My favourite way to play G is index finger on the Bass A, middle finger on the low E and pinky on high E. My ring finger gets ignored, hanging above the strings like a ninja waiting to strike. I'm stubborn and won't change, despite trying all the different ways!

Later on, you'll be switching fingers as your index finger will move and be pressed flat on all 6 strings to form Barr chords. For now it's important to just get used to chord positions and how to play them clearly, as you'll need to know the chord shapes later on. These chord shapes won't ever change, despite which fingers you'll use.

The sales guy you mentioned has probably been playing for years, stay focused on your current level and have fun :-)