guaranteeing that a sequence never skips

Am I correct to assume that SERIAL does not guarantee that a sequence 
won't skip (e.g. one successful INSERT gets 32 and the next might be 34)?

Sometimes a business requirement is that a serial sequence never skips, 
e.g. when generating invoice/ticket/formal letter numbers. Would an 
INSERT INTO t (id, ...) VALUES (SELECT MAX(col)+1 FROM t, ...) suffice, 
or must I install a trigger too to do additional checking?

-- 
dave