I don't need my dragons to be tagged with a good or evil descriptor. I need stats for different kinds of dragons that I can make good, neutral or evil at my discretion.
My players encountered three metallics in two different 3e campaigns. One Gold was a puzzle maker, who was rather insistent on the players completing his puzzles - they didn't end up fighting him but it was close. One Silver was a traitor to a circle of "good" dragons (which included chromatics), and was selling ancient demon-binding secrets to a necromancer in exchange for immortality through lichdom. The third was a bound thrall to a powerful sorceress.
So, a purely good dragon? Never. However, I'll switch a creature's alignment anytime I feel like it to suit a good story. I have never had a player who was bothered by this.