To answer the question briefly: if you didn't find a guide or enough examples of a good
practice online, then it is underpromoted (which
includes the possibility that it doesn't exist).
People are still working on ways to cite online material, especially when referring to
evolving resources like MathOverflow. If you can provide enough reference points in
the bibliography to ensure that anyone who checks will find that to which you refer,
then you have done your part of the job. MathOverflow assists in this by providing
static unmodifiable data (question number, user number, timestamp of revision, etc.)
as well as a link that gives you the data in a format suitable for bibliographic reference,
but that is no guarantee that things won't change after some transition.
If OEIS had made some suggestion for reference which you still find wanting, I offer
two: For the short term (while you are still active), prepare an electronic file which
contains additional research notes for limited distribution. This can include your
favorite online resource links or even gossip about which reference librarians to
ask, but one thing it can include is a jpeg which is a screen snapshot of the OEIS
page as it looked when you retrieved it. That way, for any future researcher who
can't verify that part of your work and asks something like "What choo talking about
Willis?", you can send them jpeg as supporting material. Hopefully they will still have
a jpeg reader in that decade.
For the rest of posterity, also include links to other papers which use the sequence.
Future historians can then infer the existence of a great and powerful database,
something like a lost continent, which was gifted to mankind for the purpose of
furthering intellectual acheivements, and which may inspire them to reconstruct
it from whatever digital archives the insect rulers left to them.
Gerhard "Needs Some New Science Fiction" Paseman, 2013.05.20