However, I'm sure carriers know when you're tethering when you're not supposed to. I know Straight Talk checks by looking at the HTTP User-Agent header. If you're looking at a website on your laptop and you're tethering through your phone in order to use Straight Talk's mobile data connection, it'll show something similar to this:
Mozilla/5.0 (Windows NT 6.2; WOW64; rv:25.0) Gecko/20100101 Firefox/25.0
That tells them that you're using Firefox on a Windows 8 machine on their network. The only explanation is that you're tethering, and they'll cut you off for not following TOS. If you were using Firefox on your Nexus 4 (not sure what Nexus 5 would show) it would look like this:
Mozilla/5.0 (Linux; U; Android 4.2; en-us; Nexus 4 Build/JOP24G) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30
So if you do tether without having an option on your plan that allows you to tether, I'd download an add-on on your computer that changes the user-agent string to your phone's so that it looks like you're using your phone instead of a computer.