I use Ms until I get a letter from the addressee then if I have to write again I am guided by their own signature block.
Mrs\Miss\Ms is only correct because we should mention all categories of men and wommen.
For instance, in a party the comperer starts only as" ladies and gentlemen". so we should mentin all categories.
I mostly use MS ... because when you are not knowing if the refering person is married or not, saying MRS may get them angry!!!!;-)