Should I start with p=reject?

Start with p=none and a rua address, read the reports until all your senders pass aligned, then move to quarantine and reject.

Do Google and Yahoo require DMARC?

Their bulk-sender rules require a DMARC record (p=none is accepted) for domains sending in volume.

DMARC Check

DMARC is a DNS policy at _dmarc.<domain> that tells receivers what to do with mail that fails SPF and DKIM alignment, and where to send reports. This check parses the policy, alignment modes, percentage, and report addresses and flags weak or broken settings.

When to use this: You are publishing or tightening a DMARC policy, or a provider rejects mail citing DMARC. Command prefix dmarc:

About the DMARC Check

Parse a domain’s DMARC policy: p, sp, pct, alignment modes and where aggregate (rua) and forensic (ruf) reports go.

FAQ

Should I start with p=reject?: Start with p=none and a rua address, read the reports until all your senders pass aligned, then move to quarantine and reject.
Do Google and Yahoo require DMARC?: Their bulk-sender rules require a DMARC record (p=none is accepted) for domains sending in volume.