The first internet forum I joined was one about photography. It was filled with mostly beginners with a few intermediate guys sprinkled in and rarely would a professional chime in.
I watched as they went back and forth on all the " rules " of photography and cropping was always a hot topic. Many said that you cannot crop just below a joint ( knee, elbow or above like an ankle or wrist ). I remember a heated discussion about cropping at the shin was an absolute mistake. These were all photographers who probably have never been published yet were setting the rules and criticizing others work.
So later that day I went to the mall and as I was walking though I noticed in the window of at least three major chain womens clothing stores pictures hanging that were breaking all the rules those forum photographers were stating. Here were pictures that were part of a national ad campaign breaking all the rules believed by photographers that will never get a picture in a national ad campaign.
" Beginners follow the rules and an artist has no boundaries " I don't know the source of that quote but I always liked it.
While there shouldn't be any set rule about cropping that would automatically dismiss it as a reject there are a few things that can be done to make the photo more pleasing.
Where you crop at the joint makes a difference and care should be taken not to them look like an amputee. A classic 3/4 shot would start at the head and crop just above the knee in a pleasing point to accentuate the thigh.
Sometimes you might have to crop something out of the background or foreground that would be distracting to the picture. In doing so your crop line might end up being in an undesirable position on the model. Then you have to decide which is more distracting to the overall photograph, bad crop or bad element.
Another factor in cropping is size constraints. If you are going to keep all your pictures the same size you are limited to some extent as to what you can achieve when cropping.
The example you've posted looks fine even though it's broken the " rule " of cropping just below the knee. There is far too much good stuff going on for that crop to be distracting in any way and it would take someone focused on it to even notice it.
When I make a gallery of model pictures I hand crop each one and every so often I'll crop one just below the knee for those that might admire that part of the leg. It's really hard to please everyone all the time but it doesn't hurt to step outside the box and throw them a bone.