You can and it will probably still work OK, for now, but WEBIDs are being deprecated by DreamHost. They are being replaced by the user numbers you mentioned. I would recommend you use the numbers, it will be more 'future proof'.
Before WEBIDs were deprecated, new users could choose their own WEBID when signing up. However, these days the WEBID is automatically created by the DreamHost system.
Now that I think about it, you must be correct. Otherwise the sign-up script couldn't check that the username is unique.
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