As an example, with Verizon if you have a tiered data plan, tethering is already included and counts against your data allotment. However, if you have unlimited data, Verizon requires the purchase of a tethering plan (at the time of this writing it is about $30 per month) and most phones will perform a subscription check prior to granting access to tethering functionality.
Within the forums, discussions of tethering and apps that help manage the functionality are fine - but discussions of ways to circumvent paid functionality are not. At Android Central, this falls under the "Illegal Activities" section of the forum rules. If a plan in question already allows for tethering and you're not trying to lie about the usage, then that usage doesn't fall under the above and is fine.
To be clear, asking and offering help to defraud, steal from or trick the carriers into getting paid services for free is the issue here and members that violate these rules will see threads closed, receive at least one warning and be infracted as necessary. I'm sure that there will be questions, feel free to send a PM to any moderator or myself to see those addressed.