Sometimes my seats disappear and sometime they don't. I never know until I call SMS. Like atl runner, we book flights nearly a year in advance. I currently have 7 open itineraries that I monitor from time to time.
And if they disappear from Delta.com, as far as I'm concerned, they've disappeared. I call immediately to have them re-instated to the website as I have lost the occasional plum seat assignment by not checking often enough.