Somewhat depending on your location, but in general wood in contact with soil means bugs. Termites are your worst enemy, but I assume you don't want to bring any bugs into your house with your firewood.

Two things you never want to do: stack firewood against your house (garage or any structure made of wood); or lay your wood directly (indirectly) on soil.

There are plenty of barriers you could use that are cheap and would offer some protection (pressure treated lumber and tarps is one way to go)