This depends: How long will you be one server?
Some things you always do.
Like backups, and backups, and - oh yeah, BACKUPS.
Preferably backups that go somewhere other than your server, and get regularly restore-tested.
Some things don't ever make sense to do for a single server:
You wouldn't run a load balancer on a single machine pointing traffic back to itself. Even if you eventually intend to have a farm of servers you would wait until you got your second machine to implement load balancing.
Other things don't make sense to do for an environment that will always be a single server:
My personal website is served from a single machine. I never intend to make it redundant, or to serve enough traffic to load balance it. Therefore I don't use puppet/chef/radmind to manage the machine.
In my case I don't even use
etckeeper - I just copy the files to a .bak when I'm making major changes - but if you want to use a tool it's perfectly fine.
Everything else is a judgment call.
If you want to learn Puppet this is a great way to do so.
If you know you'll have 10 machines this time next year having Puppet already set up and working will make new deployments much quicker ; Conversely if you know you'll be one machine for the next 3-5-10 years and you don't have the time to master a new tool you can certainly put it off for a while...