I am not sure if there are any specific rules, but I like to use one main domain for each droplet, and give the hostname the same name.

I find it helps me to keep track of what droplet I am using.

If you use a sub-domain, the hostname will be truncated at the ., so when you ssh in your will get:

user@test:~$

If you use a full domain you will get:

user@yourdomain:~$

When I am logging in to multiple droplets throughout the day, I find it helpful to be able to glance at a terminal and see that I am logged in to me@mydomain

Beautiful, thank you. I will use only exemple.com.