I’m a lefty but play the regular way. Is that “righty”? I’m not sure. Neither hand has a dominant position on the whistle. I figure that if I ever start playing a flute with keys or some other instrument with similar fingering it might be best to play the standard way now. I’m not sure the other instruments are ambidexterous.
Seeing as how the whistle uses both hands anyway, it really doesn’t matter. Keep the left hand on top. 'Twill make it much easier if you ever want to switch to, say, the flute, saxophone, or recorder.
My neighbour, who happens to be one of the best whistle players you will ever get to hear, plays left handed. Her daughter is left handed but plays right handed, both on whistle and pipes. Some of her other (5) kids are citeogs but play righthanded while one righthanded plays lefthanded. There doesn’t seem to be a system to it, except that they are all beautiful musicians.