Yes. You should always log off at the
end of your session, especially for per-minute based
service plans. If you do not log off manually, you will
be logged off automatically within 5-10 minutes under
the following circumstances:
Your computer is switched off.
You remove or switch off your wireless networking card
or disconnect the computer from a wired Ethernet connection
You leave the wireless hotspot to where the wireless
access point can no longer detect your wireless networking
card
The wireless access point is rebooted