Recently I had some problems staying under my monthly mobile bandwidth cap. One reason being an increased travel and tethering time. Although working on an up to date system is preferable most of the time, I started to turn off automatic updates on my Mac while working using my mobile connection.
The automatic update settings can be found under System Preferences -> App Store.
When traveling you can uncheck all but the “Automatically check for updates” check box. That way you get notified if new updates are available and can set yourself a reminder to update your system when a landline connection is available.