Once, during an observed lesson, a colleague was given the feedback that he needed to differentiate more. What he had done, in order to be sensitive to the feelings of the students, was pretend he’d run out of a certain handout in order to give the weaker students a different handout.
A definition of differentiation could be ‘giving different work to different students in the class based on their ability’. Now, even being as covert as possible, if this happens lesson after lesson the students are going to pick up on it: ‘Hey, why’ve I got the stupid worksheet?’; ‘What? Why are you helping them but not me?’; ‘Look at the geeks over there only doing the tough questions’. Among other things, this can damage the class dynamic and demotivate those at the bottom end.
I think self-differentiating is better. If you have three worksheets of differing difficulty levels, let students choose which ones they want to do, don’t choose for them. If you have a gap-fill exercise, let students choose whether or not they want the words that go in the gaps. If we do things this way, no student will be tacitly assigned the label of ‘weakest in the class’.