>> No, but it's a typical value with some tolerance.  On our drawings we 
>> always have stuff that is labelled like 16.68 +/- 0.20 mm, if they made 
>> all the parts with an average of 16.66 mm then some of them wouldn't fit 
>> together properly.
>
> As I understand it, the stuff you make is somewhat smaller than a 
> building. ;-)

Exactly the same principles though.

Imagine you have a rise of 3050mm that you want to meet with 10 steps, and 
you can make each step with a tolerance of +/- 50 mm.

You might specify each step as 305 +/ 50 mm, which will give you a total 
height of 3050 +/- 50 mm.  Using your argument though, you would say the 305 
is pointless and you may as well just say 300.  That would make every 
staircase 5cm out on average!

In buildings I would imagine that lots of distances add up, and if you don't 
at least aim to make each one correct, then at some point something is not 
going to fit.

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