For me, my riding stance is generally sitting on the seat, but not fully. Always stretched forward as its good for my arse not to get so sore!
For downhills and a high speed flat ride, I'm in the dropbars so I can have easy access to the brakes should something go BADLY wrong, quickly. Additionally, you do get slightly more chances to swing your way out of trouble in a smooth motion, or so it feels.
Normal riding on the flat, basically on the top of the drop so I can have access to the gear changes and the brakes and it helps to see ahead clearly. I alternative from the top of the drop to the middle of the bar, depending how long my cycle is and what I'm debating to do.
Uphill, if its a short hill that's not challenging, then I basically just power my way up the hill by building up speed on the flat beforehand and staying seated while going uphill. I try to keep myself seated as much as possible if I am going up hills, as I have found that it is best to measure yourself before you get to the hill and set a steady pace to make it up the hill and plan out the gear changes in advance. Only real time when I have to bleedin' stand up on the pedals is when I've realised that its gone all pear-shaped and I have to power the rest of the way up!
That kills.
Mind you, staying seated is hard when you're not exactly fit...