JetBlue does not overbook and we have flown with them for 15 years and I have never had them reassign my seats.Just remember that even though you are able to choose seats in advance doesn’t mean those seats will be there when you get to the airport. Airlines can and do reassign seats without notifying you.
Personally I much prefer SWAs boarding procedure. At least I feel like I have some control over where I sit and with whom.