UNIX hosts and MX "best practice"

[John H. Yates]

What is considered "best practice" for UNIX hosts these days? To
assign a UNIX host its MX record as itself, or to leave it unspecified?

I've observed that it falls back to the A record if you don't bother
specifying it, but it makes "nslookup" MX queries look "different" than
I am used to, and I was just wondering what current "best practice" is.

Thanks, John