Modify value only if node is new
The is_new property is set by the node_save function, which is the same function that will invoke hook_node_presave - so your are safe in your assumption that this property will be present.

If you were not doing this in hook_node_presave, another way of testing if a node is new is by checking if the nid property is present and non-zero:

if (empty($node-&gt;nid)) {
  // ... this is a new node ...
}