Personally i could not live with 6mm. Fret the E string at the first fret and behind the last fret on the neck, now measure the distance around the 9th fret... that is a true reading of your action height. I look for 2mm to 3mm.
If you have 6mm at the bridge end your bridge is too high. If you lower it and get a lot of fret buzz then the relief on the neck is wrong and it needs a truss rod adjustment.
It is very rare you need to shim a neck.
