I tether all the time, and while it is an awesome feature, it kills your iphone battery pretty dam fast.
If you will mainly use wifi, and want to have the option of 3g, and you already have a large data cap on your iphone, just tether.
If you plan on using 3g a lot, get a 3g ipad for the sake of the battery.
Thats how i see it, but I can tether for free and have no additional charges from my carrier. I hear that is not the case in the US