Add JavaScript and css to page with nodes only if there is no tag in any of the nodes
you shouldn't have use hook_preprocess_node as it gets fired on each node separately, so if only one of the node doesn't have that specific tag the css and javascript will be added to the page unless other nodes have, so rather than hook_preprocess_node use hook_preprocess_page and there you can find an array of nodes (use dsm function with devel module enabled to see variable structure) and you can iterate over that array and check for condition