Sprint does not have "free" wifi tethering.
If you don't have it on your account, it will simply fail to connect, assuming you're running stock. It won't let you connect so you don't get a nasty bill.
That said, if you've rommed your phone, or even rooted and patched the hotspot app, than you can use it without issue. Sprint doesn't seem to care much, but if you're really going wild with it, they may still send you a little nastygram.