If the device is truly idle then it should be 0%, period. Anything else means unnecessary wakeups eating the battery.

Of course if you're trying to measure CPU load then the device is not really idle. If you run top in an osso-xterm for example you will see non-zero CPU percentages going to top, osso-xterm and Xorg, or if you run top in an ssh session you will see top, sshd and the wlan driver instead. That's fine, as long as top is the only process that causes these. Powertop (if you can find it) may be more useful in tracking down unjustified CPU usage.