I have tethered for over 6 months and was not charged extra and at the time I was not on a everything plan but I did pay for the BB service. I have since moved to an Everything plan and I don't see any changes in my rates. Sprint can only see your tethereing as data flow which shows up the same when you use your phone anything on the network (browser, apps). They can't tell the difference (that's how it was explained to me).
I use it and I have downloaded 100 mb files and nothing came back. You should have no problem.