Roll back to Mountain Lion but don't lose saved documents
Yes, you can safely backup with Time Machine in OS X Mavericks and restore your newer files later when you're back on OS X Mountain Lion.


Complete a backup using Time Machine on OS X Mavericks to get your new/updated files into the backup
Reboot the machine with the Time Machine disk connected
At startup, hold Cmd+R to get into Recovery mode (or hold Option to select the volume to boot from and choose the Recovery HD from the Time Machine disk)
Choose Restore From Time Machine Backup from the list of actions
Select the Time Machine volume and then the backup dated before the upgrade that you wish to restore from (it will show you a list of all available backups)
After booting into the OS X Mountain Lion system, turn off Time Machine in System Preferences > Time Machine (to avoid another backup starting soon, thus slowing down the system and adding to the cognitive load on you to choose the correct versions to restore)
Browse the Time Machine disk from the Time Machine interface and pick the specific files from the latest backup to restore
Turn on Time Machine again in System Preferences