I've personally flown for UA and NW and have friends who fly for AA, DL and HA and I've never heard of any of those airlines with a policy stating that FAs have to stay seated while everyone else deplanes...what would be the point?

In contrast, I do know that JetBlue asks their non-rev'ing employees to help tidy the plan...but the two times I've flown them the non-revs have been up and out of their seats tidying as soon as the plane blocks in.