You can generally assume that the last number on anything that gives you three decimal places is a guess. If you have a very sensitive piece of equipment that gives 5 or 6 decimal places, the last 2 or so numbers are a guess.
It depends on what you're doing, and it depends on how much you care. If it meant the difference between good results and bad results, I'd be more specific, but even then, those last numbers are just a guess.
What exactly are you using? A spec? A measuring cylinder?