First off frost strike will always hit lower at arp cap than an Obliterate,unless you have BQL bite,cuz you know,double dips are still present(inb4 str dks become utterly sucky).I do NOT play str(this should be HEAVILY noted).As an arp dk you MUST bring out your available Obliterates first and if you can use KM for frost strikes since a crit>noncrit but usually that doesn't happen,usually you'll have time for ~2 frost strieks in a row and after that more obliterates.You can also make it so you hit OB,FS,OB,FS,Blood rune,blood rune(or death ruens respectively),HB,frost strike.Blood runes/death runes should be on cooldown if you intend to use HB,otherwise it won't proc 4pT10 bonus,in fact if it actually worked properly I'd likely remove HB from my spec and get one more point into Blood caked blade.For the RP you're right,in a 25 man raid you'll have all the RP you could ever wish for.So this automatically removes Chill of the grave(rp talant) from your spec.And even if you RP dump,it's not a bad thing,if everything else(FFUU runes and DD/BB runes) are down,you can and well absolutely MUST use your available GCDs to dish out as much damage as possible,whether it's a crit or not in that circumstance is completely irrelevant.
In case someone didn't bother reading what I said at the start,i'll repeat for ARP prioritising OB before a KM proced frost strike is better since OB hits for much more than a frost strike does even with frost strikes having double dips.