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init is the recommended hook and no, the rewrite rules will not be overwritten each time the hook fires or that the page is loaded, unless if you call flush_rewrite_rules() or WP_Rewrite::flush_rules() in your callback.
add_action( 'init', 'add_my_rewrite_rules' );
function add_my_rewrite_rules() {
add_rewrite_rule( ... );
flush_rewrite_rules(); // ...
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