In the television series, Supernatural S04E16, It is said that the first seal is broken when a rightious man serves in the hell. And so, Dean has broken the seal. But already, John has served in hell because of the same reason as Dean. So, John must have broken the seal and not Dean right.
When John dies, he goes to hell where he is tortured everyday by Alastair so that he tortures other souls to get himself off the rack. Later Alastair, reveals to Dean that his father never broke, i.e. never accepted his deal.
However, Dean succumbed and gave in to Alastair in just 3 months(30 yrs in human years) and tortured souls for the next 10(human) years. In S04E11 Family Remains, Dean even confesses to Sam that he enjoyed torturing souls, as he finally had the chance to dish out the same pain that he'd endured for the past thirty years.
TLDR: John and Dean are both righteous men but the first seal was not broken by John as he never gave in to Alastair's torture.