Using significant figures when measuring with an instrument with marks other than powers of ten If an instrument does have markings that are powers of ten, then finding the precision of the instrument is easy. For example, if a graduated cylinder had markings for every $\pu{0.1 mL}$, or every $\pu{mL}$, or every $\pu{L}$, then it is easy to tell that those measurements will be to the hundredth mL, to the tenth of a $\pu{mL}$, and to the tenth of a $\pu{L}$, respectively. These would be valid measurements with the correct number of significant figures.
See this image, for example:

These markings are to the tenths and ones. This is simple.
However, what if that graduated cylinder went up by something like $\pu{5 mL}$? Would its precision be to the tens or to the ones (in other words, will the measurements be to the $\pu{0.1 mL}$ or to the $\pu{1 mL}$)? What would be the valid measurement?
I think the rule of thumb for the "reading error" being $\pm\,0.1$ of the smallest division applies here, so for your case, it'd be $5\;\mathrm{mL}\pm\, 0.5\,\mathrm{mL}$. See this page, entitled "Math Skills - Scientific Notation", from Texas A&amp;M. Specifically, it is noted that


  A rule of thumb: read the volume to 1/10 or 0.1 of the smallest division. (This rule applies to any measurement.) This means that the error in reading (called the reading error) is 1/10 or 0.1 of the smallest division on the glassware. If you are less sure of yourself, you can read to 1/5 or 0.2 of the smallest division.