Stretching at the barre is necessary even when you are first learning ballet. Opportunities to stretch come in between exercises while your ballet teacher describes the next exercise. Often a teacher will give a stretching exercise at the end of the ballet barre workout, in order to take advantage of the warmed up muscles, and also to give the ballet students a brief rest from the most strenuous moves.
One set of muscles that need stretching and relaxing between each exercise is the rotator, or turnout muscles. After plies for example, turn your legs in and plie. Then lean into each hip, gently stretching all the muscles in the hip area. Do this after every barre exercise.
Stretching and relaxing muscles does not diminish your muscle tone, but does the opposite. Muscles that retain tension are weaker than muscles that are relaxed properly with correct stretching.
After battement tendu, relaxing the calves with a demi plie is advised. Relax the foot muscles briefly by pressing each foot up to demi pointe (while standing on the other) to stretch the sole of the foot muscles out.
Some ballet teachers give a stretch exercise at the barre before grand battement. This may be something like placing one leg on the barre, to your devant or front position, and stretching over it. A stretch should be held still for 15-30 seconds at least. No bouncing or pulsing is needed - and movement does not help with the stretch. Turning out the supporting leg and pivoting the body turns you into a la seconde, leg to the side.
There are two common ways you can stretch in a la seconde. One, simply hold a well aligned position (spinal posture correct, working hip level with the supporting hip if possible), and stretch sideways over the leg, holding it 15-30 seconds, or musical equivalent. For more flexible students, slide down the bar, staying close to it, and maintain the widened second position, holding turnout well, 15-30 seconds.
And this is how you can do your stretching at the ballet barre to improve your basic positions.
Get yourself a copy of THE BALLET BIBLE if you want a complete description and photos of ballet barre exercises.