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?

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dave