They really only go after people with the so called "unlimited" plans. However, some people on unlimited plans never get notified. I think it mostly depends on how many GB you use per month on unlimited. I think they start taking a good look around 3 to 5 GB for most people, but it still varies from person to person. If you have a set GB plan per month they are more than happy to let you tether and go over your plan amount so they can charge you extra & for the GB overage. And honestly, if you are willing to pay for the extra GBs it's none of the business how they were used.
Maybe others can reply with different experiences, but I've tethered with out a tethering plan many times and have never been notified about anything. Of course, I have the 2 GB per month plan and even with tethering have never gone over. But, like I said, others might have different stories to tell.
I think the bottom line is, if you are on an "unlimited" type of plan, you should probably be careful.