According to http://armenia.usembassy.gov/using_your_visa2.html:
As soon as you enter the United States on your immigrant visa, you
will become a Legal Permanent Resident, as evidenced by the I-551
stamp in your passport. Your I-551 Alien Registration Card (commonly
referred to as a green card) will be sent via the mail to the address
you listed on your immigrant visa application a few weeks after you
enter the United States. However, for a period of one year your
endorsed immigrant visa and entry stamp may serve as temporary proof
of permanent residence. If necessary, you may leave the United States
before you have received your green card, using your endorsed
immigrant visa as evidence of your permanent legal status. After one
year, Legal Permanent Residents must travel in and out of the United
States with their foreign country passport and green card.
Meanwhile, Timatic tells us:
If the passport shows a stamp "processed for I-551 temporary evidence
of lawful admittance for permanent residence valid until receipt of
I-551 employment authorized", such stamp is to be considered Form
Make sure that your immigrant visa contains the wording "Upon endorsement serves as temporary I-551" (or similar). This means that once the visa is endorsed (stamped by the official at the border), it is a temporary Green Card (I-551) for one year.
However, as a practical matter, if you're traveling by air, it could be the case that airline representatives deny boarding to return to the US because they see the expired visa and do not understand the I-551 stamp. You could refer them to page 40 of the Carrier Information Guide for this case. Getting the Green Card by mail could simplify things, but it would also cause much greater complication if it gets lost in the mail. In the worst case, you would have to visit a US Embassy or Consulate overseas to get things straightened out.
Note that staying outside the United States for more than six months in a year, or for an entire year, could jeopardize your immigration status unless you've received permission from USCIS in advance.
It's also worth noting that the immigrant visa + stamp is not necessarily accepted by third countries that grant visa-free transit privileges to US Permanent Residents. If you need to transit one of these countries, not having your I-551 card may mean you have to get a transit visa.