Time Machine makes two passes on each file. If your file is open/locked/changing, then Time Machine will skip that file for that interval. Basically, it collects a list of all files needing backup and when it gets to the file - it might skip it and save it to try again once the first pass is complete. If the second pass fails, you miss that file.
Ideally, you would quit Parallels (or at least shut down the storage / finalize all the writes) and then make a backup to avoid missing files or having a file being saved in an "unusable" state.
It looks like the vendor is changing the default behavior to accommodate Time Machine according to:
It also gets complicated based on which other storage settings you use for the virtual disk technology - and I'll leave that for Parallels support to answer in a knowledge base article - but how Time Machine works is easily tested.
Run a backup and then run
tmutil compare to see what files are different than the ones saved in a snapshot. Try restoring the Parallels VM to a new location and booting it. The problem with timing issues is you might get a sane backup 9 times out of 10 or 99,999 out of 100,000 - but it's really about how much data you can afford to lose.
Most people I know skip backing up virtual machines and implement an alternative method of saving user files off image (or syncing them) and/or setting a reminder to periodically make a snapshot backup of the state of the VM by scripting a shut down and then scripting a copy of the data files to a place where Time Machine will back up that periodic snapshot.
Once you have that set up, you can exclude the running VM files from Time Machine backup and save time/space on the backup media of incremental backups that might not actually be workable.
It looks like Smart Guard marks all the files excluded from Time Machine except the backup snapshots and the optimize scheduled snapshots automatically every 48 hours. You should be able to look at your Time Machine backups with a tool like BackupLoupe after running for a week and confirm that you are only getting backup snapshots and that they are happening every 48 hours.