Excluding Items from Time Machine Backup
It is a good idea to exclude caches, temporary files, scratch volumes, etc from Time Machine backup. Excluding such items conserves space and reduces the time to backup.
How to exclude items PERMALINK
Open Time Machine system preferences, the click on. The following dialog appears; you can drag things into it from the Finder.
I personally exclude system files and applications, because I backup those items via other means.
Adobe Lightroom considerations PERMALINK
The previews and cache generated by Adobe Lightroom should be excluded from backup, because they can be regenerated whenever desired.
Time Machine can and should be used to backup the Lightroom catalog file, but there are some caveats, see Adobe Lightroom and Time Machine Backup.
Especially with large 1:1 previews, the previews alone can total hundreds of gigabytes for users with many images. Previews are normally located in the same folder as your Lightroom Catalog, found in a folder ending in “...Previews.lrdata”.
Apple Aperture considerations PERMALINK
The previews and cache generated by Apple Aperture are best excluded from backup, because they can be regenerated when desired.
Unlike Lightroom, Apple states that Time Machine can be used with Aperture without issues on Mac OS X Snow Leopard, but not Leopard.
Time Machine can and should be used to backup Aperture (in addition to other backups!).