Apologies for the thread necro, but just to keep this discussion in one place:
Some years ago, a beginner student in our dojo asked a visiting Japanese sensei how to grip, given that they were left-handed. His response: 'Same grip'. Unperturbed, the student asked, 'But what did left-handed samurai do?', to which the sensei thundered: 'There were no left-handed samurai ... We killed them all!'
Only after a moment's pause did he let a wry smile cross his face ...
Most left-handed kendoka, iaidoka, and aikidoka I have met confirm my own experience, and what someone posted above: being left-handed, but using the 'standard' grip is an advantage, as it's the left hand that does the 'power' work; also, if the sword needs to be used one-handed, the stronger left arm is better suited to being closer to the end of the tsuka, and the weaker right hand is better placed closer to the tsuba.
Mark T