If it's bad enough that I tell our FA to sit down, then I would make sure that the passengers are seated as well. Otherwise, I consider them advisory in nature for our passengers, with the exception of below 10,000'.

That said, it's never been an issue for me. The pax are smart enough to stay seated.

As a side note, we turn the signs off during the night on international flights when the passengers are sleeping since they shine down on the beds in the back of the plane.