Is it muttar for my wife to wash dishes after shabbos lunch (or after 1st day yom tov lunch)even though the dishes will not be needed for shabbos- if it really bothers her to have dirty dishes.
i.e. can we say that she is not doing hachana but rather it enhances her shabbos or yom tov
Shulchan Aruch (OC 323:6) and Mishnah Berurah (323:26) write that it is prohibited to wash dishes when they are not required for used on Shabbos itself. This is the reason for which dishes are not generally washed after Seudah Shelishis. However, if the household norm is for the sink to always be clear of dishes, and the sink being full of dishes is seen as clashing with the honor of Shabbos, then it is permitted to wash the dishes. Pots, however, may not be washed, but only dishes that do not fall into the muktzeh category.
Sources: Rashi (Shabbos117b) writes that the prohibition of washing dishes is on account of preparation for weekdays. This implies that if the washing is not for preparation for weekdays, the act would be permitted. [There is certain debate about the position of Rambam in this matter.] Maharshag, Orach Chaim, vol. I, no. 61, writes explicitly that this is the case, and permits washing dishes for the sake of hygiene and cleanliness; Tzitz Eliezer, vol. 14, no. 37 concurs with the ruling.