You will find that some boards have a bigger sweet spot and you can move around a bit while paddling..but some have a very small sweet spot and you only have to be an inch out of place and it all feels wrong.
I also like to have my feet as close together as I can..I think if you have them too wide..and close to the rail of the board you only make the board want to tip over more if you get wobbly.
But then if the going gets rough I have to take on the wider stance or else I'm in the water..

As far as forward and back position...I find that there is a perfect spot..where the nose isn't to high and the tail isn't too low..and visa-versa.
You can usually see this correct position when watching some one else..so don't be afraid to ask some one.."Hey is the tail of my board too much out of the water..or is water flowing over the tail area as I paddle"..It's not easy looking back yourself.
Notice this pic of Champcrow's brother having his first go at SUP...
It's obvious from this view that he is a bit too far forward on the board..click here

Also...I have found that a slightly staggered stance works best..It gives a better forward/back stability..and lets you get to the surfing stance quicker because you're almost already there...Once you get used to this position it's hard to go back to a parallel stance.
You will find that something on the board helps you remember that correct possition..some even put a sticker on the rail..I can look down and see where my foot is in regard to the hand hole on the deck.
One more thing (sorry for my long winded answer)..I find that if I'm heading into a wind I move ever so slightly forward..and If I'm going down wind I move back a little.
After many hours of practice you will find your own sweet spot on your own board..Here's a pic of mine on my 10'6" Naish.
DJ