Are women mechuyav in washing mayim acharonim when benching?
In principle, there is no difference in the halachah of mayim acharonim between men and women. However, the common custom is that women do not wash mayim acharonim, whereas many men do. The reason for this could be as follows:
According to several rishonim, there is a full obligation to wash maim acharonim. The reason for the obligation is becasue of the danger involved in a salt called melach sedomis. The obligation is set down in the Gemara (Chulin 105a). In addition, one may not recite birkas hamazon with dirty hands (Berachos 53b).
Today, however, several rishonim (primarily Tosafos, Chulin ibid) opine that there is no obligation, because our salt does not have the dangerous potency of melach sedomis. In addition, because it is customary today to eat with cutlery, hands are generally not sullied, and there is no need to wash them before benching.
Shulchan Aruch (OC 181:1) writes that one must wash mayim acharonim, yet mentions those who are lenient (ibid. 10). Several poskim concur that it is correct to be stringent (Shulchan Aruch Harav 9, Gra, Birkei Yosef, Aruch Hashulchan 5, Ben Ish Chai Shelach 6). This ruling would apply to men and women equally (see Halichos Beitah 12:2; Halichos Bas Yisrael 3:5).
However, the custom is that women do not wash mayim acharonim. Although poskim write that it is correct to be stringent, the obligation of mayim acharonim remains a safek, and as a safek in a rabbinic obligation, there is room for leniency. The reason for which many are stringent is on account of kabbalistic grounds, which generally do not apply to women. Shevet Halevi (4:23) also writes that mayim acharonim today is considered a chumra, which women have not accepted on themselves.
שבה”ל ד-כג שהיום המנהג שאין נוטלות, וכן בהליכות בת ישראל פ”ג הע’ י”א
שלמת חיים השלם קעז, ורש”ז בהליכות בת ישראל שם , וכן הרב אלישיב זצ”ל בהערות חולין קה,ב, וכן בוזאת הברכה פי”ד שחייבות לפי שני הטעמים.