My take on this is that a DME/GPS arrival is a series of steps and depending on what quadrant you are in, as long as you remain above the step you can maneuver safely with the required terrain clearance.
A VOR/NDB is a series of tracks which have been designed to steer you safely on a course to remain clear of any terrain, provided you remain on track within the accepted tolerances. Therefore it would make sense that you must track to an IAF such as on the DME arc or for the straight in approach once you are within the 25nm MSA to intercept the track for a safe letdown. As there are no steps in a quadrant with the said approaches but rather a series of steps on a particular track I would assume that this would not be an accepted practice. I'll have a look in my jeps when I get to work tomorrow and see what I can dig up.