Excellent question and one that I've grappled with. Here's what I've settle upon as my guidelines.
1. When to start your own NEW review thread
a. If you want to. This is an open forum after all.
b. If the review you're looking to share is long, contains important observations or commentary, or deviates from other prior reviews of the same cigar or cigar line. You generally don't want to overshadow the original poster's review if at all possible as it might be considered it a bit immodest. For the record, I've been guilty of this in the past.
2. When to add to an EXISTING review thread
a. When you are responding directly to specific points or impressions the original poster or other posters made.
b. When your experience with the cigar or cigar line is very similar to that of the original poster and you simply wish to affirm this on the public record.
c. When your review is short, or does not provide much detail, observations, or commentary. This last point is not meant to diminish these capsule experiences, rather, it refers more to considerations of website resources and ease of searching and researching.
I've considerer another practice that I think I'll begin using and that is to cite links to other reviews of the cigar or cigar line. I normally conduct this research prior to posting my review anyway and I think it would help the reader if other experiences were collected for easy comparison or to provide counterpoints. I see that a number of reviewers already do this.
Wilkey