It depends on how steep the slope is: a rise of more than 1 in 10 will probably need at least a few steps to make it more comfortable (mentally at least) to the people using it. If you have a physically challenged friend, or other potential such visitors, you may want to design a meandering ramp, to allow easier access. Steps or serious switchbacks will probably need some form of retaining walls to keep the garden from burying the path after a heavy rain. Railings might be needed, too, to prevent a tumble. I would read up on it thoroughly, and--when in doubt--hire a good landscape designer.

Also, in a Michigan winter a slope could get slippy.