What happens when there are fractions of points?
MageRewards does not support point fractions.
When there are fractions of points MageRewards will round to the nearest whole number. Generally this should never occur since rules are only based off of whole numbers of points. The only obvious scenario is when the store's dollar currency changes and the conversion is not a whole multiple of the base currency.
If you're looking to display points as a fraction of points, like 5.23 points for example, you can modify the appearance of the points to make it look like there are fractions of points by copying app/design/frontend/base/default/template/rewards/points.phtml to your local theme directory and modifying as needed.