Our company has such a small contingency fuel in the fuel plan (5 mins) that its not 'whether' you take extra fuel or not but, how much extra will we take? A little or a lot.
Senior Dispatcher. I've never discussed how much fuel to take with a dispatcher. It must be different where you work. As far as defects go though, I once had the dispatcher inform me that we were tech, due to some not-very-obvious issue. Issue closed as far as I was concerned. Off to the bar!
My take on the fuel issue is simple: Want to choose the fuel? Fly it yourself!