The Community of Disc Golfers and About All Things Disc Golf
It depends. In a tournament the TD decides what the teeing area is. In a casual round the group decides.
The general rule is laid out in the PDGA Rules which deals with natural pads (dirt) and Tee Pads ( some kind of "improved" surface like cement or rubber or carpet or wood chips). For a dirt pads you must be within 3 meters behind the line. For a Tee Pad you must be on it. However in bad conditions, typically for safety reasons, some TD's will call "Liberal Tee Pads" allowing you to tee off from the side of the pad.
I have played in tournaments where the TD's refused to allow Liberal Pads, leaving the weird situation where the teeing area was the worst place to drive from. In wet conditions a dirt pad often turns into a mud pit or even a submerged mud pit.
There is an art to dealing with bad tee pads. In my area most of the courses have big, flat cement pads (which are great so long as they are not covered in ice). But if you get too spoiled on good pads you may be poorly prepared when you visit a course where the pads look like mini moon craters.
I was throwing with a guy on Friday who did this. The tee pads were pretty decent. A wood frame with a light gravel base. He did his run-up off the side on the tee box on the grass. He said he didnt like the footing of the tee boxes. Said something about the (uneven) gravel. I threw from within the tee boxes with little trouble. Strange~
I personally don't think gravel is a good material for tee boxes. It creates a very movable surface for turning an ankle or sliding on the gravel.