Please note that when dealing with high volumes of bandwidth (ie, 1000's of gig's), it's quite common that the count in whm is not accurate. You may want to check with your datacenter to see if they have mrtg graphs that you can see for the most accurate count.
This is very true! The stats shown in Cpanel is built only on web traffic for
each account and doesn't take into account default IP traffic to or from the
server not going to a specific site or any other services or processes that
might be installed on your server outside of Cpanel also using traffic.
However, setting the accounts to auto suspend is still not a bad idea but
given what PlatinumServerM just pointed out (and I agree), you might want
to set the limits on each account to something slightly lower than what
they really need be set just as a margin of safety for yourself.
Other than that, keep a close eye on your traffic reports, system logs,
and provider server traffic information and you won't have any surprises
and will know how much is being used and if you are getting close to
your server bandwidth limits.