I agree with the 1/n chance of going for the sweep providing you can pull the sweep off after the first kill. You almost have to be certain that you'll be successful based on the first kill odds because if you still have a 1/n chance of winning after your first kill, you're fighting a losing battle. Basing your decision on 1/n will lead you into trouble because that chance is only for one event when you really have to calculate for more than 1 event.
I guess what I'm getting at is if your chances increase (hopefully dramatically) after your first kill you're in a much better position to go for the sweep and 1/n chance of winning is viable. I believe this is why people tend to set themselves up with rules of 1.5-2x starting attacking troops which increases their chances of sweeping after the first.
I think a statistician could come up with a cost/benefit analysis in terms of points to gain and what you can lose if you miss your target. It's an interesting question and I'm sure there's a proper way of coming up with a percentage based on the number of players you're up against.
The number of troops you can deploy after your first kill is a massive factor. I think looking past the first kill is the most important facet of calculating the viability of a sweep. I've seen so many new players (in standard games) hand the game over to the next player because they didn't have the troops to continue their sweep after their first successful kill.
There's also the number of spoils you have when you turn in (be it 3, 4 or 5) and the number of spoils your first target has (5, 4 or 3). If you have 5 spoils of your own and you're going for someone with 5 themself, you have the best chance of turning in twice. This may be enough reason to go for a 1/n chance of going for the sweep...
Variables such as the number of troops/spoils of your next target, and set worth must be considered when doing your calculations. I don't think there is a clearcut answer to this question because every situation is different. I completely agree with Jippd about the battle odds calculator being your best friend. Having the odds allows you more leeway to actually go for the win instead of using rules such as 1.5-2x the number of troops (a little restrictive). I think having "rules" about going for the sweep can really impede your chances of taking more games. Waiting for the stars to allign (having requirements met) may be a good strategy against players who are using the same strategy because you're essentially in a Mexican stand-off but (I assume) when you're up against the higher ranked players, these requirements will arise less often as the better players will take their opportunity if the numbers stack up.
my two cents