MX Records - good or bad practice to use "domain.com" ??

Hi! Just wondering what are best practices for setting up MX records.

I see in many tutorials that "mail.domain.com" is a standard setup.

Unfortunately this requires getting a separate SSL certificate when using encryption.

Is using "domain.com" considered also good practice? This would allow me to reuse the same certificate I have for the website.

Or should this be avoided for some reason?

I am running Ubuntu 14's with Virtualmin.

Many thanks!