Many great responses so far and different ways people use & store their files, it`s hard to say any method is best and all seem to have there own pros & cons
Keeping your files indefinitely is obviously the safest option, you will always have every file there if you ever decide to change something or even use the original files as part of a different project, however you may never use them again so potentially just taking up loads of drive space
Then the delete immediately or soon after you have made your project, this method obviously means you could end up one day wishing you had kept that footage, but on the other hand if you are fairly sure you will never need again, you are not only saving drive space but making it easier to keep your files organised with no needless clutter
So far the poll is pretty 1 sided with most people voting indefinitely