Salvation and rewards are given on a different basis, and we should be careful not to confuse them.
Salvation is about God’s work for us. Rewards are about our work for God.
When it comes to salvation, our work for God is no substitute for God’s work for us. God saves us because of Christ’s work, not ours.
Likewise, when it comes to rewards, God’s work for us is no substitute for our work for God. God rewards us for our work, not Christ’s. (Of course, it is empowered by Christ, but God nevertheless refers to it as our work.)
Christ paid the price (eternal hell, in a finite period of time, which only he could do) for all our sins, once and for all (Hebrews 10:12-18). If we have trusted him for that provision, we will not pay the eternal price; that is, we will not go to hell.
He has fully forgiven our sins and we are completely secure in the love of Christ (Psalm 103:8-18; Romans 8:31-39). Our salvation is sure, and we will not undergo the judgment of condemnation (John 5:24; Romans 8:1).
But although the forgiveness of our sins has every bearing on our eternal destination, it has no automatic bearing on our eternal rewards. The Bible teaches not only forgiveness of our sins but also consequences of our choices. These consequences apply despite our forgiveness. Forgiveness means that God eliminates our eternal condemnation. But it does not mean that our actions in this life have no consequences on earth. (Forgiven people can still contract an STD, go to jail for drunk driving, or suffer the death penalty, for example.)
Neither does it mean our choices have no consequences in eternity. Forgiven people can still lose their rewards and forfeit eternal positions of responsibility they could have had if they’d served Christ on earth.
With our salvation, the work was Christ’s. With our rewards, the work is ours. It’s imperative that we trust in Christ, lean on him, and draw upon him for power, for apart from him we can do nothing. But if we hope to receive a reward, we must still do the necessary work. As our forefathers put it, we must bear the cross if we are to wear the crown.
Belief (trust, faith) determines our eternal destination: where we will be. Behavior (obedience, works) determines our eternal rewards: what we will have and do there (treasures in heaven or works burned up, ruling over five or ten cities, etc.).
Works do not affect our destination (though they should be a product of it, Eph. 2:8-10), since our redemption is secured by the work of Christ. However, works do affect our reward experienced at that destination.
Just as there are eternal consequences to our faith, so there are eternal consequences to our works.
For more information on this subject, see Randy Alcorn's book Heaven.