Going back to 2009, there is a forum discussion of this issue at http://www.telerik.com/forums/two-way-rating-precision. The response at the time indicated that a change was coming, but I have since learned that this change never happened.

If I set Value="2.5" and Precision="Item", the Value is rounded to 3.

It should be possible to allow users to rate only a whole number of stars (Precision="Item"), but if the Value is set to a decimal, it should display the fractional result.