I agree when people say carioca and paulista accent are widespread, but they are NOT neutral in any aspect. I can easily recognise where people came from when they are speaking with me for the first time. Every city will have its specific accent and, in fact, it doesn't matter for being understood. If you learn brazilian portuguese with carioca accent, for example, we will understand you, but definetivelly know your teacher is from Rio de Janeiro. Just that. One thing I can say to you, if you learn portuguese with a brasiliense (people from Brasília, the capital) people will understand you perfectly and will not detect where your accent is from. It is not neutral, but undetactable, if you know what I mean.