Hi Baz,
Dustins pretty much on the money. Having your feet more or less "Duck" will be determined by how wide your stance is, the wider your stance, the more toe out you should have .... to a point.
Too wide a stance with not enough toe out can lead to knee pain so it is worth checking it out and experimenting. Having your feet further apart will definately make landing tricks easier but going uncomfortably wide will make you less comfortable and may cause ankle issues depending upon how tall you are and how wide your feet are apart.
Wave riding stance is usually dictated by the board if you are riding a surfboard and is also a factor of how powered you are riding. Riding unstrapped in smaller surf or in more marginal conditions is better as you can shuffle your foot position to trim the board for optimal wave riding.
On a twin tip you would expect to be powered so a comfortable stance is probably best with a bias towards slightly wider than your shoulder width.
To determine your natural stance. Take a normal step forward and stand with your arms by your side, don't think about foot position or stance, just place your feet where they naturally want to go. This should give you an idea of your natural "duck". Width is usually shoulder width being ther centre of your footstraps, an inch either way will be OK, but lean towards going wider.
Hope this helps,
