I'll usually start off with A or D. For me, it's a way to see if it's even an option for the owner of the card.
I had this one experience where I contacted the owner of a card (sketchcollectors) to see if they were willing to sell. The owner of the card replied with it's not for sale. I replied that I'm interested in it and if you'd ever want to sell it could you keep me in mind? Also, I asked if they'd mind if once a year I contacted them briefly to see if they'd change their mind? They replied "no problem". So, once a year I'd send a quick little message just to see if they changed their mind and after the 3rd year they replied with "yes" I'd be willing to sell and I now own it.
I think as long as your polite about it there's no harm in asking about a certain card. We're all collectors and things change in our lives and so you never know until you ask. IMO