The file slack is that portion of the file that got allocated, but never actually used because of cluster sizing... so you might have allocated 512 bytes (physical size) for some file X, but only have used 490 bytes (logical size). The term "slack" refers to difference between a file's physical size and logical size so the 22 unused bytes constitute the slack (just so we agree on terminology). In the grand scheme of things, erasing the file slack (even if you CAN), doesn't "buy" you much.
So as for your question, I guess your answer depends on what you want, but TYPICALLY you would erase free space (wipes out data stored in files you've "deleted from disk" by writing some pattern of bits to those locations) though you might also want to erase the pagefile (erases the contest of the virtual memory that exists in the pagefile( s ) on disk) on shutdown. The older version of Eraser provided the ability to define a blank erase pattern which can prove VERY helpful when dealing with virtual machines. If you overwrite the pagefile with 0's for instance... that compresses very nicely when you Zip the virtual machine's virtual hard drive.
Unfortunately it LOOKS like the ability to define a blank pattern doesn't exist in 6.x :cry: