I believe as you do that when we become a Christian, all of our sins, past, present and future are under the blood of Christ. That being the case, what will we be judged for in heaven with all of our sins forgiven and forgotten?
Believers will be judged in heaven after the Rapture at the Bema Seat judgment, or judgment seat of Christ (1 Cor. 3:10-15, 2 Cor. 5:10). This judgment will not concern our salvation and is not about sin. It concerns the works we performed during our life on Earth as believers and will result in various crowns being distributed to those deemed worthy. Things that were done due to the Lord’s prompting and in His strength are called good and will qualify for rewards, while things done out of our own will and in our own strength are called bad and will be burned up in the fire of judgment. That’s the only judgment we’ll ever face because as you say the Lord already paid the penalty that was due us for our sins (Colossians 2:13-15).