However, girls who have very tight legs and tight joints and ligaments must focus on pulling up the leg in order to improve the alignment of the leg, so it really does depend on what kind of body you have.
There is a fine line between 'pulling up' and 'snapping back' the knees, and it is a difficult area to control for many individuals. Hypermobile individuals must focus on lengthening the leg, rather than overly contracting the quadricep to 'pull up' when straightening the leg, to prevent the knees from going too far back. Many dancers feel that they have to bend their knees, however, this is not the case. You do not have to bend the knee, just make sure that it is not 120% straight! This is also very important when walking normally, not just in ballet.