Yes, with a Nexus you get to tether without anything special on your plan's end.
Don't have to root on a Nexus. There's no carrier firmware to remove.
T-Mobile "detects" tethering by seeing if you're using a desktop or mobile browser UAstring. It seems to be relatively random whether or not they try to check in on your usage, but if they decide to, you'll see your browser redirect to a hotspot information page if you're on a desktop browser.
Problem is that the Chrome for Android browser also shows up as a desktop UAstring when visiting sites, so they've started to shy away from this method because they're selling their own devices that are either compatible or preloaded with Chrome.
Long story short is they can detect browser UAstring whether you're using the built in tethering or some root app or anything else, so it doesn't matter what you do. Just tether away and if they redirect you, that's something you can manage when (or if) you get there. One quick fix is a browser UAstring switcher you can apply to Firefox/Chrome that'll mask it. Works.