i think starting a relationship based on truth and with a clean slate is always better, but then again thats my opinion. I would let my future wife know everything and i would expect the same from her, as far as HASHMAT`s comment goes, well a man would not like to hear about the past yes that is true but dont you think it is better you find out earlier from the person themself rather than finding out somehow through someone else and then ruining something nice. Hopefully i make sense.
I think I agree to you. I'd much rather tell her in a casual manner if I ever liked someone and why it didn't workout rather than have her find out through someone else. But again this depends upon the person as well. If she's cool and understanding then it won't be a big problem.
However, I think its always a bad idea to tell your spouse about your feelings/crush/affair for anyone who is still in our life i.e. a current colleague. People who were there in the past are not right there, but someone your spouse knows you are around and had feelings for at one time can make him/her really uncomfortable.
Bye the way, I would really like to know married girl's perspective on this one.