Unfortunately the specification does not tell anything about precision. The xml scheme definition states that it is of the type xsd:decimal:

<xsd:restriction base="xsd:decimal"> 
  <xsd:minInclusive value="0.0"/> 
  <xsd:maxInclusive value="1.0"/> 
</xsd:restriction> 

I have a sitemap generator that uses up to 10 positions after decimal point. Where often only the last few positions differ. These numbers are perfectly right according to the xsd, but yet i found some pages(3,4) that state that only 0.0, 0.1, 0.2, .., 1.0 are valid values.

How will the search engines react to such a sitemap? Will some just round the value?

I know that it is unlikely that someone can provide an answer to that question, unless he works for that search engine, but i think experiences will also do.