Do I need to wipe anything before restoring a full nandroid backup? I'm used to wiping everything when installing a new ROM, but I would like to know if a nandroid restore automatically wipes out the partitions it is recovering.

When I run a restore, I see some messages saying it is "formatting cache" and "erasing boot before restore", but the rest of my partitions (system, data, .android_secure, etc) don't have any accompanying messages about them being cleared first.

Does a nandroid restore overwrite partitions, or does it merge the files with the current structure?

And in the interest of making this applicable to a wider audience, is the process different for different phones (some have extra partitions) or different versions of clockworkmod? Or is the process the same for everyone?
I tested this, and your concern is valid.

I added a dummy file to /system and then did a nandroid restore.  The dummy file survived.  Repeated the same test on /data with same result.  So I don't know why CWM doesn't wipe those partitions first.

The CWM author probably made an assumption it doesn't matter or there may be a valid reason they don't get wiped.