Steve,
Not for Weight & Balance purposes, as the DOW is associated with DOI. DOI will always be the same for a given aircraft, but LIZFW (Loaded Index at ZFW) will differ depending on how the aircraft's been loaded.
Hence, you'll start with DOW and DOI, then add the weight of the payload to get ZFW (if, indeed, the flight is carrying payload which is not always the case).
For aircraft which requires ballast fuel in order to trim without payload (for instance MD11 or B727), the ZFW will include the ballast fuel.
So is ETW = Empty Tank Weight (and thus the same as ZFW?) or Estimated Take-off Weight, and if it's the latter shouldn't it then be ETOW?