Actually, no. If you're going stock to stock, it should NOT wipe data.

Does flashing stock rom delete everything?

Based on my experience with the original Droid, flashing a new rom usually calls for wiping system, cache, data, boot, and dalvik. Internal memory data is deleted, but external microsd storage is not cleared.

Do I need to wipe data before flashing rom?

Yes. Do not clear internal storage however.