If you create a temporary partition, you are creating a disk image file. if you make a temporary partition that is, say, 300 megabytes, you have a 300 megabyte image file sitting on your hard drive.
When you are done, Toast asks you if you want to save or not. It is not asking if you want to save the temporary partition. it is just asking if you want to save a list of files you burned to CD.
You always save the temporary partition when you make one. If you want that hard disk space back, you must manually go and delete the disk image file.