I think it is a good idea. I started a 150 gallon about 4 mos. ago. I keep a quarantine tank going all the time. I buy a fish and quarantine it. When I move the fish over to the display tank, I buy another for the quarantine.
When I finish my display, I am not sure what I will do with it. Some people say they put it only when they need it. Others leave it in case something happens in the display tank, they have something ready.
Hope this helps.