Good question.

Short answer... it probably doesn't matter ;) Most film, and modern film workflows, have enough exposure latitude that if your exposure is out by a fraction of a stop, this becomes basically insignificant.

But, since you asked, there are accepted (rounded) ½-stop and ⅓-stop increments for the ISO scale, for the f-stop scale and for the shutter speed scale.

For f-stops, the standard ½-stop scale is...

full stop   -½-stop
1.0         1.2
1.4         1.7
2.0         2.4
2.8         3.5
4.0         4.5
5.6         6.7
8.0         9.5
11          13
16          19
22          27
32

The standard ⅓-stop scale is...

full stop   -⅓-stop     -⅔-stop
1.0         1.1         1.3
1.4         1.6         1.8
2.0         2.2         2.5
2.8         3.2         3.5
4.0         4.5         5.0
5.6         6.3         7.1
8.0         9.0         10
11          13          14
16          18          20
22          25          29
32

It might be interesting to note how this is physically implemented... When you turn the aperture ring, you are setting the aperture to any arbitrary size along the scale. They've just decided to incorporate detents at the full-stop and half-stop locations – but you don't actually have to use the detents if you don't want to. Again, I think it's not too important to worry about absolute preciseness here – who's to say that the detents are precisely where they are supposed to be?