My arm exercises are progressing quite well but my right bicep is starting to out distance the left bicep by 7 lbs. I do 5 sets of curls with the last set to failure. Should I modify my routine to help the left keep up with the right or just keep going to failure on both biceps and worry about it later?

Use dumbbells. Use the same weight for both arms. Do no more reps with the stronger arm than you can do with the weaker arm. Over time, the strength will even out and both will then continue to progress.