WHy not, so I know?
This is from USCIS site. Note, it clearly says "show your permanent resident card" or I-551 stamp. It does not say "Welcome Notice." Also, they do refer to the Welcome Notice as something one will receive, not something one may use as a substitute for his green card. Here is the authority:
If You Recently Became a Permanent Resident
After you become a permanent resident we mail you a welcome notice. Then we mail your new permanent resident card (green card). If you move before you get your card, call customer service at 1-800-375-5283 when you move.
You should also call customer service if any of the following occur:
You received your welcome notice or card, but you believe there is an error
It has been more than 30 days since you became a permanent resident, and you have not received your welcome notice.
It has been more than 30 days since you received your welcome notice but you have not received your new card.
You have questions about your new status as a permanent resident.
Applying for other documents from other agencies (Driver's License, Social Security Card) as a Permanent resident
Permanent residents can work in the U.S. When you were granted permanent residence, we may have placed a special stamp in your passport or given you a temporary status document. Until you receive your new Permanent Resident Card, use this stamp or document if:
You apply for an unrestricted social security card
You apply for a driver's license
You need to travel abroad for less than one year
When you apply for a job, show your new permanent resident card (green card) or your unrestricted social security card, and a valid identity document, such as your driver's license to prove eligibility to work here in the U.S.
For information about how to apply for a social security card, please see the Social Security Administration website. If you were already in the U.S. when you became a permanent resident, and have not yet received temporary evidence of your new status, please follow the instructions on your approval notice.
I hope that clarifies the issue. If have some other reference, please let me know.
Now, if temporary evidence of your new status means Welcome Notice, then I stand corrected. I believe a Welcome Notice is not such a document. Where would we find a definition of temporary evidence of status? Here is something I found and the document is not a welcome notice:
(g) Eligibility for evidence of permanent residence while in deportation, exclusion, or removal proceedings. A person in deportation, exclusion, or removal proceedings is entitled to evidence of permanent resident status until ordered excluded, deported, or removed. USCIS will issue such evidence in the form of a temporary permanent resident document that will remain valid until the proceedings are concluded. Issuance of evidence of permanent residence to an alien who had permanent resident status when the proceedings commenced shall not affect those proceedings. (Revised effective 11/28/11, 76 FR 53764)