When to use Certified Mail

[Arnold Finkelstein]

I'm a little foggy on when you must, or should, use Certified Mail with return receipt to correspond with a taxpayer.

At present, my understanding is that you must use it when you hold a Certificate and wish to start the 6 month evictment eligibility clock.

However, I deal mostly with deeds and it's not clear that I need to use CM w/RR at all.  Thoughts?