Mormonism's belief is not that you will be rewarded because some one posthumously baptized you, but because you accepted that baptism in the after-life. All are rewarded according to their own decisions/desires.

Edit:

Well actually, Mormonism is a Christian faith that teaches that salvation comes in and through Jesus Christ, so it really isn't your actions or their actions- but the actions of Jesus Christ- taken on your behalf, that leads to salvation- if you accept it. But it isn't so much a reward as it is an act of mercy.