Hi Skinjob,
A scratch set has a setting of 0 basically...if you can see it like that. This means that if a tape is in there, it has 0 data, or data that can be overwritten.
When a tape is in a Scratch set, it's available. So by moving your tape to that scratch set, you're making it available. If it has data on, its going to get overwritten if it's picked by the system.
Once in Scratch, there is no need to wipe the tape, as the system will do this automatically for you.
Laters!