The D606 is non-directional. In fact so are most of the popular choices such as the K270 and SR244.
ALL of them will slope after awhile when used on the front. You'll see this as an angle worn in the lugs in a direction opposite of tire rotation. It's primarily caused by braking forces and will be far more pronounced and will happen much sooner on disc brake front ends than with drums because discs have no "feel".
The key to getting a zillion miles out of them is to reverse them on the rim as soon as the sloping becomes apparent, then run them a few more thousand miles.
Lather, rinse, repeat.
The stock front and most other directional tires will do the same thing, but since their tread is directional flipping them on the rim isn't a real wise option.