Glad it helped you.
No you only need to do it the once usually, the only time you would need to do it again is if you update all your box art etc. then you would need to refresh the cache in that same menu to show the new art, then you would probably need to force cache the new images, what you have done now will keep until you refresh your images.
Edit:
If you look in your Launchbox/Images folder you will see the cache folder, so you can always delete the folder and the force cache again if you need to for any reason, Launchbox will automatically create the cache folder again when launched.