When it comes to liquify, less is usually best. Unless you have a model or client who specifically asks for heavy shaping, chances are you are going to have an unhappy client if this tool is overly used.
Proper posing is essential to boudoir; however, there are times that liquify will still be needed. Use of this tool in this genre has become a bit overdone and unnecessary, especially in the loving your body movement of today's world. I recently came across an image where liquify was done to the extent the client was unrecognizable. Using liquify in my own opinion should be done to shape only when wardrobe might have caused some issues (unless your client is asking for more).
My client here had a piece of lingerie that pushed in a bit when she curved the body. Most of the time, I see the photographer pushing in the skin instead of just pulling out the garment. The difference is you are now reshaping her body, rather than reshaping the garment.
This tool is not always about pushing in, but should also be about pushing other shapes out in order to get the same result without altering the client's body. Bra and garment issues can be difficult to fix when sitting compared to the better fit when laying or standing straight up. Use a small brush and lower pressure to help achieve the results desired without warping a client's abdomen or other body part drastically. Sometimes, the client is looking for the less is more approach.
Reshaping the garment has a more natural look in the final image. This can work for most wardrobe issues I have come across. Many images can work well with both reshaping the wardrobe and the skin in small increments rather than a drastic change for body shape that I have been seeing all over as of late.