When I save PNG's from the Windows 7 Snipping Tool, they save at 90-something DPI.
How do I make it save at 72 DPI?
Snipping Tool uses the DPI size of your Windows 7. So, it depends on your system's current DPI size.
At the moment, you must be using 96 DPI. You can change it to 72 DPI from Custom DPI Settings. Just follow the instructions on this page.