Disable iDevice backup when using WLAN sync

Hi everyone,

when syncing an iPhone/iPad/iPod iTunes will automatically create a backup of the device which is nice. I usually sync via USB which is reasonably fast. But sometimes I use the option to sync wirelessly which is fast enough only if you skip the backup. I know that you could disable device backups using

defaults write com.apple.iTunes DeviceBackupsDisabled -bool true

but I would love to have the option to only disable backups while syncing via WLAN. Is there a way to do that?