I found this question while researching Anne Sexton's suicide for a research paper I am writing, so I know a thing or two about her.
The answer is, quite simply, yes. She had always written about suicide and had attempted it once before. Suicide was always on her mind.
She gave her last book, The Awful Rowing Towards God, to her publisher with the specific instructions not to publish until she died. She waited until after she had made all the appearances she was scheduled to make.
Also, she wanted to die like her good friend Plath, without disfiguration. She thought Hemingway was extremely courageous but did not want to go via bullet in mouth.
I'd site my source but I am currently trying to find it again.
So the answer is, yes. It was planned very well and she even waited until all her business was in order before she went.