Are you talking about 'Barnicle', 'Wireless Tether for Root Users', 'PDANet' or the hotspot work around method? All of the above will tether your computer at no additional charge except for the last one... but only if you don't perform that one correctly. And you would know if you hadn't performed it correctly because you would be directed to a sign up page before they billed you for the (hotspot) service.
I have used all of these except 'Barnicle'. I ditched my internet service over two months ago and have enjoyed free internet through my phone ever since. First through PDANet (tethers through a USB connection), then through 'Wireless Tether' after I rooted. Everyonce in a while I'll do the hotspot work around method, but honestly I don't see much of a difference between it and 'Wireless Tether'.
Have fun.