Pier foundations at one time were the standard. While many Lenders frown on homes built on pier foundations, sheds are a differenct animal. If you are concerned about varmits go ahead "skirt" the shed. If insects along with critters are the a concern, gather up a bunch of rocks and fill in between the pillars. Alot of the old houses around here have that thing going on.
The real old house didn't even have pillars. They would take two flat rocks, placing one on top of the other then hang the joists off of them. Water doesn't wick from rock to rock and as long as there wasn't a flood, you didn't have to worry about water damage.