For years, I have never left unknown surnames of female's empty. I have always used [husband's surname] in square brackets. It is very annoying to me to have 10 Elizabeths with no surname. with the [ square bracket] I can work with the family. We had quite a discussion on this several years ago. Every one has their own way of doing this. So there are other choices in this forum on this subject.
Nettie, back in the dark old days I employed the same method as you described either with the square brackets in the Surname or Suffix field however I personally find use of Alternate Names much more useful in this respect.
Say your Elizabeth had three marriages but you were still to discover her Birth Surname, using Alternate Name she would appear at the top of the index with a blank Surname so you know that is work to be done, but she would also appear in your Index as Jane White, Jane Brown and Jane Grey if they were her three husbands Surnames which I find very useful and various female life events get recorded under different Surnames.
I do a one name study on a surname all spellings of which begin with "Surg" and can end with "iner", "inor", "ener", "enor", "nor", "eoner" and others depending on how the scribe recorded it. Different life events could be recorded with different spellings and these appear all over the index making visual identification difficult and often missed. However "enor" is the most prolific Surname ending so I add an Alternate Name 'Other Spelling' with the Surgenor spelling to each of the other spelling variations, I think TomH might have a script for this?
That means if I have Show Alternate Names turned on for the index I only need to search for 'Surgenor' and I will find all spelling variations of the Surname grouped together as if I only used one spelling. So apart from Married Names if you were working Smyth and Smith you could apply one spelling as an Alternate Name and both Surnames would now be intermingled in the Index with Show Alternate Names turned on and NOT when Show Alternate Names is turned off.
It may not be for everyone but I dumped my old antiquated Unknown Surname systems and embraced Alternate Name once the feature became available.