I have never had an issue requesting sauces to be left off or on the side (don't usually prefer sauce on steak). Have never had an issue substituting sides at Jiko, and Yachtsman specifically tells you that you can substitute any of the specific accompaniments (potatoes mostly) or sauces with another. However, you cannot sub one of the accompaniments with one of the sides that they list separately on the menu, or at least that is what we have been told, so for instance if the filet comes with au gratin potatoes and the porterhouse comes with mashed you can request the filet be served with mashed but you cannot request that the au gratin be substituted with the side of mushrooms if that makes sense. I know that I have also requested mashed potatoes at CA grill instead of whatever was supposed to be served with my meal, have not asked for substitutions at any other signature but they likely will be just as accommodating for your wife.