If there's traffic on the lists, wait until you've seen a few postings so that you know what is considered acceptable, what level of discourse and detail is commonly used, etc. If there's not I'll be a little bit annoyed. Some people actually use PARI-GP to get Real Work done, and don't spend all their time posting to various mailing lists, you know. The Fine Manual then be prepared for sarcasm. Use meaningful Subjects.
Some people get hundreds of mails every day.
The more to the point your Subject line, the more likely it is that people will proceed to read your message. Someone may have gotten bored with the old one but might get all excited about what you have to say. Don't post and mail. If you respond, on one of the lists, to someone who's obviously also subscribed, do not send a Cc to that address. If you reply directly to that person, then do not send a Cc to the list.