Is it safe to back up a disk image file when it's mounted?

Hi -
I use some small, encrypted ".sparsebundle" disk image files to keep my diary, banking info, etc., on. When I'm at the computer, I keep them mounted all the time.
Is it safe to let Time Machine automatically back up the .sparsebundle disk image files, even while they are mounted? Or would there be a possibility the backups would be corrupted, if the operating system hadn't written out cached data or something to the disk image file?
In other words, if I save any open documents on the mounted disk image, does the disk image file on disk always contain the fully-saved document and a consistent file system? Or could there still be some data cached in RAM, that would not be saved at the time the .sparsebundle file was copied? Is it always necessary to unmount a disk image before duplicating its .sparsebundle file?
Do I need to turn off Time Machine's automatic backup, and just do manual "backup now" after unmounting the disk images? That's a major pain in the behind... I really want to just let Time Machine do it's hourly backups as usual, but of course there's no point if the backups are corrupt.
Of course my own guess is "to be safe", I should unmount them before backing up. But again it's a big pain if it's not actually necessary. Does anyone know for sure?
Thanks...

My understanding is that Unix, generally, doesn't write everything out to disk immediately. There is caching in RAM to speed up disk access, including modified superblocks, modified inodes, and delayed reads and writes. Now, if you ask to read an individual file, the kernel makes sure all the cached data is written before it returns the read data. But if you try to do, for example, a block copy of the whole filesystem, the kernel doesn't know that, and there's no guarantee or expectation that the filesystem will be in a coherent state.
That's the purpose, for example, of the "sync" command - to write all this cached data to disk before unmounting, shutting down, etc., and, that's why the OS gets so upset if you just unplug a firewire drive. Because even if you've saved and closed any open documents, they haven't necessarily been fully written out to disk yet.
So what I don't understand, is how people are ever able to back up mounted disk image files without them getting corrupted. I mean, you'd think they would get corrupted all the time. It would be the equivalent of just unplugging your firewire drive before you made a backup of it, or of doing a block copy of a read/write mounted filesystem. There ought to be a very good chance of the filesystem being scrambled. So, why does it not happen?
Is there something different about the Apple filesystem, or Apple disk images? Or, maybe we're only talking about delay times in the cache of fractions of a second, so as long as you haven't written anything within a few seconds of the copy, and don't write anything during it, you're ok?
Even if Apple reps won't answer my question, the kernel and most likely all the low-level disk i/o and filesystem stuff is open source. Surely there's someone out there who would be knowledgeable enough to give a definitive answer? I was hoping to find someone like that here... I do appreciate your taking the time to answer my post, but maybe I need to go elsewhere, on some developers' mailing lists or something? Does anyone know of such a place I could start looking?
Thanks...

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