To answer your question, no it isn't bad for your car to keep it in as high a gear as you can while still maintaining speed. As long as you are above idle in RPM and your engine isn't lugging, you aren't doing any damage.
See this link for more information about lugging.
You mentioned that you might do this to be quieter and for fuel efficiency. Your car will certainly be quieter, so this is one advantage. As for fuel efficiency, there is such a thing as being in too high a gear in certain situations. Because of the vacuum created when the throttle is closed, and it will be close to closed at such a low RPM, you could potentially be making your engine work more to maintain speed than you would be if you were in a lower gear. This would mean you wouldn't be driving at optimum fuel efficiency. Of course, this depends on your car and many other factors, so there's no way to be sure being in 5th gear is worse for your fuel economy than being in 4th gear at such a low speed. If I had to guess, you will still be getting better fuel economy in 5th gear at 1,000 RPM.
When in doubt, to maximize fuel efficiency, use as high a gear as you can manage for the power required.
