I don't know that this is a question that can really be answered, to be honest. Moralists of the time certainly noted the contrast between prescriptions to be modest and look chaste and the reality that women of fashion were constantly pushing the envelope in terms of dress and behavior. (That said, these moralists had just as many problems with women wearing too much lace, expensive fabric, trained skirts, tight stays, etc. as with low and/or wide necklines.)
At the same time, modern perceptions of the sexy neckline of this period are often more informed by film costuming than historical reality: modern films tend to ignore anything that covers the bosom unless they want to depict a character as prudish or frumpy While elite women at court, at home, or at formal balls might expose quite a bit of cleavage:
Beatrice de Cusance, Princess of Cantecroix and Duchess of Lorraine, ca. 1635
Princess Maria Clementina Sobieska, 1719
Isabella Blackett, Countess of Buchan, ca. 1743
Queen Charlotte with her two Eldest Sons, 1764-69
... this was not the norm. For the poor and middle-class, it was important to display the appropriate amount of modesty to help keep their position as respectable women, and to protect them from the sun. Their gowns would generally be cut in the same way, but the neckline would be filled in with a partlet or collar (in the seventeenth century) or covered with a handkerchief (in the eighteenth). So it's not the case that women on the whole went around with exposed decollétage.
Captain Browne Bushell's Bride, Elizabeth, née Fairfax, 1633
Mrs Salesbury with her Grandchildren Edward and Elizabeth Bagot, 1675-76
David Garrick with his Wife Eva-Maria Veigel, 1757-64
That said, the situation is not black and white. Elite women did wear collars/partlets and handkerchiefs, particularly the latter - but they had more opportunities to leave them off, and to decide to be painted without them.