I used to leave the surname blank when I didn't know the maiden name of the wife. Then I ended up with a couple 100 women with first names only and it was a challenge figuring out which person I was working with. Then I started using "married to Smith" in the surname field. Again all the women ended up at the top of my index and made it difficult to see them in relation to the family I was working with. It also caused all kinds of problems with the little green leaf for these individuals.
I finally found a solution that works for me. I enter the married surname in the surname field and in the suffix field I enter (married) in parenthesis. This allows the woman to show up in the index under her married name. The little green leaf is happy because it has a surname to search (Ancestry search engine does not use the given name in searches the last time I checked). Using the suffix field also exports well in gedcom. My reports and other items also include the suffix field so I can easily see that I have entered a married name for these individuals.
Once I find the maiden name of the woman, I change her surname and remove the suffix notation.