I've started to search for and catalog big brand DMARC records to look for ideas and suggestions, and also to develop some best practice recommendations.
One thing I'm seeing quite often is that a big company will put "p=reject" and "sp=reject" in the same DMARC record. In this scenario, the "sp=" setting is actually not needed-- it is extraneous.
The "p" setting is for your choice of DMARC setting. The "sp" setting is for your choice of DMARC setting for any subdomains. If you don't set "sp" then the "p" policy is applied to any subdomains. So the only reason you would want to add "sp=" is if you want to specify a different policy for subdomains. If you want to give this domain and any subdomains the same policy, you don't need to include the "sp=" directive.
In short, there's no need to add "sp=" unless you want subdomains treated differently. Why would you add the "sp=" setting? If you don't have any legitimate subdomains, you could set your domain policy to "p=none" (safer for the main domain) but "sp=reject" (more restrictive for subdomains) to tell the world that any subdomains seen should be bounced (because they wouldn't authenticate properly, because you in theory don't have any subdomains).
Here's an easy guide to the variables present or optional in a DMARC record. This seems worth bookmarking.