In my opinion it's healthy. What you hear, is not exceptional for a CB500/550. If it annoys you, adjust idle to not lower than 1200 rpm. A perfectionist could opt for these steps:
1. have an identical dwell for 1+4 and 2+3.
2. identical timing for 1+4 and 2+3 within 1,5 - 2 degrees of eachother
3. precision synching the carbs.
Personally I don't give a d@mn what the idle sounds like, as long as my engine starts humming at > 2500 rpm.