Well,
as long as I know, that heaps of garbage gets saved in the temp, I feel free to keep it and delete whatever I like.
Nice about the temp-files are the masks, that are generated automatical. It is - for me - a good way for partial renders.
And it made the making of a tutorial very easy ,-)
One thing that I dislike is that terragen is forced to pause work as soon I start 'Explore temporary files' from the menu.
Volker