It would be a good idea to not send usernames and passwords to email addresses on sign-up.

What are you talking about?

If you're talking about network registrations, you'd want to talk to them. The only mIRC registration is when you purchase it and there's no reason not to send your registration information to you if you paid for it.

It's probably about this forum.
After you sign up you receive a confirmation e-mail that contains your password in plain text.

Given that this is nothing but a forum and 99% of users here have nothing but post access (which any user is free to get), "security" is pretty relaxed. As with anything on the internet, you should use different passwords for different sites, which should keep you safe

Reply posted to the wrong person. Sorry.
If you do get a password via e-mail, then you should change it to something of your choice as soon as possible.

If the site doesn't offer/allow you to alter your password, then I would delete the account and stop using it.

Worst case scenario (that I can think of right now), is if the site didn't allow you to delete your account. This can be, depending on the laws of where the site is located, a legal violation of your rights. This is one of the main reasons why you should read all of the documentation when signing up for a site, no matter how boring and/or repetetive it can be/seem.