We are installing pergo flooring on the second floor of our home that is under construction. Does putting fiberglass insulation between the tji joists actually help the sound transmittance? (i heard that the sound is transfered mainly through the joists and I don't want to waste any money if the difference is minimal)

Fiberglass insulation will do very little to stop the transmission of sound from the floor above, as most of the sound travels through the common solid surfaces that the ceiling/floor are both connected to (floor joists, screws, etc). If you don't want to hear anything from above, you need to "float" the upstairs floor, which can be a big project. I've seen this done by laying out neoprene blocks on the sub-floor, then placing 2X4s on top of the neoprene and using that as a frame to put another sub-floor on top of. It's also important to isolate your floating floor from the walls, because sound can travel from the floor down the walls to the room below. If there's loud noise throughout the upstairs room (as opposed to the sound of stomping feet or people bowling on the floor), some of that sound will travel down the walls anyway. The easiest solution is to carpet the upstairs room instead of putting down Pergo. This won't stop all of the sound (especially from a loud stereo, etc) but if you use a thick carpet and thick pad it will greatly reduce the sound of heavy feet & hard shoes stomping around over your head (or beer bottles falling on the floor).