If you are more used to Imperial measures (inches not centimetres) and your copy of iWork came with metric units activated as default, you can change it in Preferences>Rulers. Preferences are under Pages, Numbers or Keynote menu.
Click on the Ruler Units drop-down menu and choose the measure unit of your preference.
It's useful not just for when you want to use the unit that you are more comfortable with, but for finer adjustment of tabs and bullets when formatting text. Instead of using split inches or centimeters, you can use typographical points, which are more suitable for paragraph or bullet indentations.
And the other way round, when positioning graphics objects on a page, split inches or centimetres may give a more exact position to the object. Set the coordinates in Metrics Inspector.
There is a converter widget with the Dashboard. Click on Dashboard in the Dock, choose units from drop-down menus, enter numbers and convert.