Your wife is your manager, she asked you to do something which you really don't agree with, what would you do?

Well, there is no option in this case, the relationship here have be to be ignored, the only thing to take care about it is the professional, if you are in a meeting and each one is discuss his opinion so no problem if they were disagree , but if she gave me an order, so it should be obeyed because the organization hierarchy give here the power and the authority to run the org.
and give orders.

Prepare well and seek a meeting and discuss it out without showing any signs of personal like or dislike but explain some feeble advantage of what is suggested already and then present and defend the better proposition with advantages what you wish to propose as better option.

Since she is the manager(and may also be the boss), there is no option than to do what she wants to be done, being that it's a key role of her post to take necessary decision(s) and full responsibilities of such decision(s).
Work ethics and family life shouldn't be mixed up if even it will affect the former negatively.

Simple - do as directed, unless you are a partner in that business that has contributed financially or in other ways to the business and you may feel that decision may harm the business, you can express your mind irregardless whether the manager is your wife or not, although ultimately as the manager she will have the final say but is she is a good manager she will at least listen and respect your opinion even if she does not agree with it.

This is why, we promote the focus-on-work philosophy J now if the situation is such then to follow the order is your job either you agree or disagree. You could discuss the issue with your boss but if she wants to follow it then do or die. But don’t worry,  I am sure, at home  you’ll prove a rude hubby ….

put aside your personal relationship and with the professional relationship, as a subordinate you can make your boss understand the pro's and cons of the things asked to do and if still insists something which has got no value addition, you can just do and give it her as a boss you being a subordinate :-)

سأخبرها عن رأى ثم اقوم بتنفيذ ما تطلبه مع تحملها جميع التبعات

As your superior, the qustion of not agreeing to directive does not arise. We have to differentiate between matrimonial and official relationship. But if you are obliged and find yourself in situation like this occur, you express the objective behind your opinion highlighting the  benefits those opinion will bring to the organization in terms of progress and development.