Common sense says that flexibility is called for, and safety says that a 'no later than' point be established by you if not already dictated in Standard Operating Procedures. It's silly not to use the rubber speedbrakes a little early if you're hot and high, and patently unsafe if you take gear down so late that you're unsure if there's time to achieve 'Down and Locked' indications too late for a stable approach.
Above all else, if you do forget to put 'em down, make sure you put the gear lever down before you leave the wreckage - it looks much better to the investigators.