There's no reason to create them all as individual e-mail accounts. You can just set up forwarders that lead to a generic
site.admin@ account and set up a filter for that in your mail client to keep the noise isolated.
You'll want them, because:
webmaster@ is where good natured people do things like report broken links, it happens more than you'd think!
abuse@ is nice to have, it sometimes keeps people from going directly to your host / data center / bandwidth provider in the event that someone found a way to use your server to send SPAM.
postmaster@ is handy to check, and make sure root's mail is also sent there, at the least it will show you if your mail server is configured incorrectly, it will also catch bounces that let you know you have a spammer.
I usually check my 'catch all' once a week and clean it out. It only takes a few minutes. Once in a while you find a real gem, which is something like "Your site helped me, I just wanted to say thanks for the resource!" .. which always makes your day :)
This, of course in addition to the convenience of automatically generated SSL certificates - SSL providers will send confirmation email to one of the above addresses, so you need to be able to receive it.