You'll seriously make your life easier if you pay the $15/month. JMO.
The only time I tether is either in an emergency or when I can plug my Nexus One to the laptop to keep the Nexus One charged, and plug the laptop to the wall to keep the laptop charged.
If you tether wirelessly, you will have an iPhone wearing down its battery by talking with a cell tower AND talking wirelessly with the iPad. I doubt you'd get two hours if you walked into it with the iPhone battery at 100%.