In Traditional Yang Family Tai Chi Chuan stance work the legs should be held "naturally straight"; both legs if in Preparation stance, the back leg if in bow or empty stances.
What this means is that the leg appears to be straight but is relaxed enough not to be stiff, with no feeling of the joints being "locked".
To feel how this works I have my students stand in Preparation Form, then I ask them to push downwards with both legs until they feel their knee, ankle and hip joints "lock", then to relax their legs only enough so that they feel their knee, ankle and hip joints "unlock".
This slight "bend" to the knees and ankles (and hips but you can't see those joints no matter what position they're in) isn't clearly visible, it appears to an untrained observer that the joints are straight however this loose, relaxed, natural straightness of the leg joints conveys a feeling of springy energy to the entire body.
If someone pushes or presses into you when your joints are "locked" out straight you will simply collapse, or maybe even fall over, as there is nowhere for the energy to go. If they press or push into you with your joints "naturally straight" you can easily bend the ankles and knees and roll the hip joints as much as necessary to allow your body to "coil" and store the energy they are giving you, then you can release it back into them or neutralize the energy to the ground, whatever the circumstance calls for. (First comply and bend, then engage and extend)
By the way, I also advise my students to search for this same feeling of "natural relaxation", or Sung, throughout their entire body. Doing so follows the same method as finding it in the legs. First extend your entire body as far as it will go until you feel all of your joints "lock", then slowly and without forcing anything allow your body to relax to the point where you feel each joint "unlock" itself into a naturally straight position.
Try it, it feels great.
I hope this helps.