I have that $7/month on-device internet from my Rogers Pay-as-you-go phone (EDGE), that I tether to my N800 right now. And indeed, this is how I optimize my tethering - I limit myself to mobile sites, no downloads, etc. So far, so good, Rogers hasn't said anything - 5 months and counting.
Data usage incurred on ineligible devices or incurred while tethering (using device as wireless modem for computer) or incurred using non-Rogers (3rd party) applications downloaded to your device will be subject to pay-per-use charges of 5 cents/KB.