The forward movement of the tines makes for greater comfort for the gardener. With wheels and tines moving in the same direction, you can usually just walk beside the tiller and guide it with one hand.
If you want to do some serious tilling, having the tines turn in the opposite direction to the wheels will really, git 'er dun. However, it is more work for the tiller, forward movement is less stable and walking directly behind the tiller is usually necessary.
I have an 850 tiller from Sears. Forward/reverse tine options are helpful. My gardens are essentially level but any surface irregularities (and rocks :/) cause me some trouble when I've got the tines in reverse. Shifting to forward makes things easier but I will go over that ground multiple times to get it cultivated as well as I could have done with the tines in reverse.
Trade-offs
Steve