During a training course with Uma Dinsmore Tuli, I pledged never to give an instruction to ‘tuck the tail’ in a yoga class. I was surprised because I was familiar with this common instruction in standing poses, especially in ‘chair’ (utkatasana). But tucking the tail can place a burden on the pelvis and exacerbates our tendency to shorten the pelvic floor by sitting down on chairs and sofas. Instead we need to release through more walking, sitting differently (on the sit bones not the tail) and in our yoga, practising squats and techniques to support and release the pelvic floor and not continually tightening and shortening. Pelvic floor issues can lead to difficulties in birthing and of course, the dreaded, unmentioanable stress incontinence.
This article sums it up nicely and talks in much more detail about tail tucking and pelvic floor health.