Ideally, your knees travel out directly in the same line as where your toes are pointing, and your toes should be pointing out at somewhere between 20° and 45° from a line drawn perpendicular to your torso straight out in front of you.
If your knees are collapsing in, yes, it is poor form. It indicates that you have relatively weak leg abduction. (Remember, abduction is any movement that draws a body part away from the center line of your body.)
Collapsing knees are bad form; a little bit is not particularly dangerous to you, BUT strengthing those abductors will improve the total strength in your leg by efficiently using all muscles. Definitely work on keeping those knees out over the hips. Also, try mixing up your stance by going a few inches wider than you usually do, and you will find that you're getting more glutes and more inner leg muscles into the action. Which is a good thing - because ONLY the squat can really tax these muscles.
Collapsing knees also tend to happen in conjunction with weight too far forward. As you collapse in, notice that you tend to pitch forward. Try various mental cues to fix this, such as, "Sit back", and "Squeeze out". Practice impeccable weight-in-heels form with your warmup sets and try to carry it into your work sets. If you can't do the exercise without knee collapse, then maybe you should take off a few pounds - or stay put for a week.