If you're on FlyerTalk, you probably know how to tell if a bin is really full. If it is not, or you don't, leave it open.
If there's still room for a small bag or purse then I leave the bin open. If it is really full, I close it. I'll wait until boarding is complete then, if there's room, move my under-seat bag to the overhead.
I don't leave a bin open for a possible jacket placed on top of the bags. Someone with a jacket should know that it can fit in an otherwise full bin and will re-open it.
The FAs have enough to do to prepare for departure without having to close every bin the airplane themselves.