Using to, cc, or bcc: if a large group and you don't want to start a big problem (filling servers, taking too much time, disclosing all addresses), use BCC:

• This is polite to others on the list so they don't get spammed inappropriately as the thread goes on, or as computers are compormised, disclosing addresses.
• Also, reply-all storms can turn into trouble of interruptions and server space, especially if with large groups or attachments.
• Some people use filters based on if they are in the to: list or the cc: list or bcc: list, to decide how urgent it is to them personally (to help manage time)
• Also, bcc: politely hides the effective "to" list, so that everyone on the list does not start getting spam from the others whose computers become compromised.
• In some ways it depends on whether it is more polite to show everyone who else is on the list, and the risk that *they* will then email *everyone* repeatedly.