I read on some blog (I don't remember the URL) that having email@example.com and firstname.lastname@example.org in addition to main email help to prevent our mails from being categorized as spam. Is it true? Is it worthy to create two additional email accounts. On my hosting, only 5 are allowed. Please suggest.
It would appear so. After a quick search I turned up a few links that described why it is important to have.
To be honest I've never heard of such requirements with any of my recent web development projects, nor have I even had clients use those exact emails. Even at my company we stick with emails on a real name basis because abuse & postmaster are so generic that they're essentially spam magnets for bots since it's a default term.
If you wanted to play it safe you always could use an alias and forward it to a main account or just delete all email through there. From my experience, even at the server level on Linux, when I setup Dedicated/VPS DNS Servers I've used my primary company email so I'm atop of any issues.