Cleanliness apart, it all depends on what they`ve eaten, and when. Some spices and seasonings (particularly those in some asiatic dishes) are pungent enough not only to put one entirely off dfk, but also to make reverse oral somewhat challenging, and I remember one w.g. who must had have a lunch flavoured so generously with garlic that it virtually oozed out of her pores and when my nose approached the target my stomach began to heave.
So if it were a choice between a civvy who`s had fish and chips, and a w.g. who had vindaloo for lunch, then it`s the civve every time (and vice versa of course).