In my company we're urged to take as little as possible, but I know of many who take an extra tonne routinely and I very much doubt if there'd be any repurcutions for taking a full load every time if that was what was desired. It's just that most captains understand the commercial/environmental/common sense considerations as well and tend to justify any additional uplifts of fuel - taking extra fuel to suit their own comfort thresholds. The vast majority are happy with flight planned fuel unless there is a reasonable justification for more.. Reasons of course include fog/thunderstorms/snow/bad weather in general, expected delays, expectations of longer routing etc. etc.