Currently, if a query has a property restriction of the form "property = x", and the property is indexed in a Lucene property index, the estimated cost is the index is the number of documents indexed for that property. This is a very conservative estimate, it means all documents have the same value. So the cost is relatively high for that index.
In almost all cases, there are many distinct values for a property. Rarely there are few values, or a skewed distribution where one value contains most documents. But in almost all cases there are more than 5 distinct values.
I think it makes sense to use 5 as the default value. It is still conservative (cost of the index is high), but much better than now.