Schumacher at Mercedes uses the Brake Bias Lever to ballance the car much more than any other item. However at Ferrari he did use it more latteraly in 05 and 06 he only did so about once a lap. With the Mercedes cars he needs to change bias about 3 or 4 times a lap to get the optimal lap time. Jenson when he was at the team under the Brawn name was the pioneer of this and he did so a couple of times a lap. Rosberg and Rubens use the bias lever, however not as much as Michael or Jenson.
The Mercedes lever is usually a 5 posistion lever with the the level towards the front of the car being as much as 70/30 towards the front and the rear one being 40/60 towards the rear. However this changes track to track. Each posistion is pre determined on the Mercedes car in order to to get a optimal lap time.
As for Fernando, he is a Diff Jockey, he uses the differencial entry, whitch is the same as a Diff Deceleration control, it controls the engine braking that he gets, Fernando uses the brake bias lever, but only as a small tuning device, he uses his engine braking more than brake bias tuning. In his Renault days he didnt use it as much, but with the Ferrari cars he uses it more. McLaren didnt entertain him this way, as the McLaren didnt have a great rear end on it then, and Fernando would have spun alot, and McLaren dont like you using multiple engine braking maps.
Its all down to driver style, Vettel is more of a straight up lets race guy, Hamilton is the same. However they do change maps and diff settings, however they do so when they are told to, they dont have as much freedom in this regaurd, However the freedoms they are given is where they go for tyre maps, Vettel and Hamilton have maps avalable for them for whitch stage the tyre is on, and this changes the diff accordingly, however they seem to change every 2-3 laps, and dont change their multi map or brake settings every lap.
Its no surprise as the best racer this season is Alonso, and over a race distance he is the quickest due to his active changing of diff settings. However when it comes to a single lap, Hamilton and Vettel are the best.
Its all down to driver preference really, and how he works with his team to extract the most from his car over a single lap and over a race distance.