I noticed a couple of years ago that the image cache folders on a couple of sites just became bloated as images were added and removed over time as the older cached images remain. This was in the main caused because I had a gallery used on every page of the site (not a large site though) which caused image cache to be created per page
This caused me large problems on one site in particular when the inode limit was smashed (the .txt file of the inode structure alone was a massive 50mb)
Also on that site as well the disk size was constantly hitting its limit (which obviously caused other issues as a result) and the only way to stop this from happening and managing was to run the script every night.... On that particular site I went from having to micro manage it every couple of days to empty the image_cache folder to not having to touch it at all
That works for me and am happy to flush the image_cache folder every night (it doesn't hurt to do this and I find it can help the user experience as they will rarely see an old cached version - it should always be the current image version) but you can set it to whatever interval you wish.
