https://maps.assessor.lacounty.gov/m/ That's the website I use. You can click around to bring up each lot's details in the sidebar. Washington east of Fairfax/Culver City limits definitely should use the prefix to differentiate from East Washington addresses. The west prefix reappears in the addresses of lots (seems like most, but not all lots) west of Sawtelle to Del Rey/Culver City limits. I would caution against depending on street signs because I've found errors (e.g., misspellings, using the wrong suffix, etc.) on street signs and I use that county assessor website as my reference point.

As for Washington Blvd. west of Del Rey, you're right. That part doesn't use the same grid numbering system as the rest of Washington Boulevard, so I'll remove the prefix. I have no idea how I missed that.

Best,
CTS