Remember though, it is against AT&T Rules, and if you go over the 5GB/month limit, your account will be flagged for review. Then an AT&T Rep will review your usage too see why you went over, and will notice tethering.
This is not confirmed though, because some people say AT&T cannot tell the different between Mobile browsing and tethering. My personal opinion: They can. Solely because they only allow certain ports through a mobile device, and if they notice a continuous connection from a port used on laptops, they get suspicious.
If AT&T does decide to take action, they will cancel your service, then charge you a termination fee for each line on your account.
Ofcourse this entire situation will not happen over a single month's usage. Just letting you know to not make tethering a habbit.
Good Luck!