Possibly true for some things, probably wrong for other things.
I suppose budget and use count as factors in the decision.
Future availability certainly counts (within limits as Izzy points out). If you expect to be able to replace something easily then keeping spares is not as critical, if you expect it to be hard to replace then having an extra one or two matters more.
If it is something important, hard to replace and easily lost then Nighthiker's formula becomes the "Orders of the day."