I searched the doc but did not see any method to set the default value of a property. Although this can be accomplished by using setAllNodeValue(defaultValue) before assigning specific values to nodes (works also for edges),
it won't work when new nodes are added to the graph.
I got my answer, I thought I should report it.
The setAllNodeValue (same for edges) actually solves my problem. Contrarily to what the documentation says, the method actually sets the default value for all existing and future nodes of a graph.