So, it's a bit tricky to say. While fashion was certainly more liberal for women in the Tang than say, during the Han, most of this gets reflected in hairstyles, jewelry, and head coverings (i.e. dropping the cloak/hood fashion that covered the face in favor of hats or hoods that did not cover the face). Lots of this can be attributed to the far reaches of Tang trade and influence as well as power dynamics within the empire over the dynasty's lifespan.

Typical dress for women would still be what you'd probably picture as stereotypical Chinese/Korean/Japanese dress, something like this with a little more emphasis sometimes being placed on the breasts via positioning of the fabric (having the fabric around the breasts flair out, for example) and a little more emphasis sometimes being placed on shoulders, which were, at times, laid bare. All-in-all, these (apologies for the poor quality picture) would be the common cuts in women's clothing. The low-cut and "dancer's costume" should be lumped into the same category really, as dancers of the time for court and events are noted as emphasizing the breasts (either through low-cut fashion or even by having exposed breasts) and/or emphasizing sleeve movements. The western style one is part of the range of Persian fashions (such as felt) that had found their way into Chinese society (even Uighur fashion played an important part in the post An LuShan rebellion Tang).

So as a bottom line: Were such low-cut dresses a thing in the Tang? Most certainly. Were they as ubiquitous as some media likes to claim? Not necessarily. Fashion for women of the time is more often discussed as a matter of body shape, jewelry, hairstyles, makeup, etc. rather than having clothes that expose a little more than before (but again, it was still a factor). The traditional gown styles and increasingly more and more "western" (central asian, really) styles were far more common.

I hope someone with more experience in Chinese clothing history will pop in and answer this more fully, but judging by these portraits dating to the Tang dynasty, it doesn't look like the gowns are particularly revealing (by modern Western standards, at any rate).

Looking at the images I linked to above, I'd assume the portion of the gown that wraps around the torso was probably placed above the breast (like a woman does when she wraps a towel around her). The stills from "The Empress of China" show the wrap placed at nipple-height and something is pushing the breasts up, like you'd see if the actresses were wearing a corset. To my eyes, it looks like yet another film industry attempt to sexify period clothing for a modern audience, but they got backlash from China's censors as a result.