Do you avoid replying to all when you are only BCC'd?

If you receive email via BCC, other recipients are not aware that you were sent a copy so it's actually a VERY BAD idea to Reply All in this case. It's ok to reply to the message you were BCC'd on, but it's not ok to send your reply to the people on TO or CC list. Only the sender should receive replies from BCC'd people.

This extends even further to the person using BCC in the 1st place. Generally when using BCC, you should always pause to make sure you are doing the right thing.

9 times out of 10, you will find that your motivation for using BCC is something less than fully candid... and it's almost always better to use CC or forward instead.