I do think it's prudent to replace them someplace in the 200-250K range (about half the engine life before a full rebuild). Of course, it depends on the overall condition of the vehicle and how long you expect to drive it.

My 300D is on it's third timing chain and second oil pump chain at 416K miles. I replaced the timing chain around 200K (just because) then it was damaged by a vacuum pump incident and replaced again. The oil pump chain was replaced with the oil pump before 400K miles (the pump failed and the chain was in very tough shape, it would have broken soon).