Do I need to wipe anything before restoring a full nandroid backup?
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.