Question: Do I need a public SIP domain if I wish communicate with the outside world in the future? (Lifemeeting)
Yes. You'll need to use a SIP domain that is publically resolveable e.g. yourcompany.com. This is because you'll be creating public DNS A and SRV records to enable things like open federation (if you enable this feature) and automatic sign in for your remote users who are enabled for the public SIP domain.
Question 2: What names do I need to use in my certificate if I have a Public SIP domain on the Internal ssl certificate?
I suggest using at least servername.domain.local (your AD domain) and servername.yourcompany.com (for your publically resolveable domain) but you can add SANs for sip.domain.local, etc. For consistency purposes, this ensures that both users configured for the domain.local SIP domain and the yourcompany.com SIP domain can log onto the same OCS server.
In terms of determining the SIP domain you're going to use, your best bet in my opinion is to use the SIP domain you'll be using externally from the start if you know you're going to be supporting external access.