I think the usual wide angle lens looking from below is fine, it's a simple way to enhance their butt and it actually looks nice, it's a nice POV, but once you start using Photoshop and liquify tools on photos it's just foolery, because now it's like you have 2 or 3 difference asses.
In the middle photo I shared you can clearly see how smudged the quality on the edge of the right cheek is compared to the rest of her skin, it was clearly messed with
There's no reason why you should do this. I mean maybe there is, maybe people don't care and still pay.