I think people like to use prior entry clearance if they've ever been bounced from the UK before for whatever reason, or if they think that they might be bounced when they get there (i.e. they're staying for an extended period of time, and don't have a job or house or schooling to prove ties to the US, all of which would make an IO suspicious). Prior entry clearance can give some people assurances that they won't be bounced from the UK after making the flight out there, since it's all sorted out in the US, and if you're refused you don't waste a ticket and time traveling out only to get sent back again.
But, for only a week's holiday, I don't think you'd need prior entry clearance, as long as you're honest with the IO, have ties to the US, sufficient funds for your trip, and a return ticket. A week is a reasonable time to stay, and as long as you're honest with the IO about your intentions to see your fiance and you have something to prove you have ties to the US, they'll be more than likely to let you through. These holidays will eventually count towards your visa, since it's proof that you two have met before, which is important for fiance visas to establish a genuine relationship.
So, as long as you're honest, and you have the sort of stuff I mentioned, you *should* be okay. You'll be more likely to be bounced if it looks like you're trying to hide something, even if you're not intending to abuse the system and overstay a visa.