That's up to you. Keep in mind that joining a channel with hundreds of users could cause lag when you try to /cline every single user in the channel, where the address book option produces no lag at all. Just something to keep in mind.
As for the ircops, again, use the address book. Get the nick and/or address of the ircop from /who and then add that nick to the address book in line 1 (or at least somewhere above the mode colors if used, as I explained why order is important). That will change the ircop's color in all channels. And, again, it's faster than having to /cline the user in every channel.
I'd recommend clearing the address book colors every time you start mIRC and then resetting them so that it's always accurate. You'll be doing a /who every time you (or someone else) joins a channel, so you'll be updating it anyhow. No need to keep it there and perhaps have it go out of date.
