I haven't read the whole thread thoroughly, but wanted to throw in my $0.02 on the original question, I hope I'm not interrupting...
I found recently on the expressway while pulsing and gliding between 60-75 mph that my 5th gear synchronizer started to give out.
It didn't like being shoved into gear at 60 mph from a transmission stand-still (out of gear).
So I began to leave it in 5th gear and just press the clutch, while my synchronizer still isn't totally gone.
At slower speeds (40 mph), it doesn't seem to matter, so I take it out of gear when coasting.
Just my $0.02.
Chipsndukes