i like to start at 68, let it rise to 72 or so during the active phase, and then as soon as the bubbles begin to come more slowly i bump it up towards 80. the extra warmth towards the end of fermentation gives them the little boost they need to finish dry.
Another great trick for brewing these beers: the ratio of sugar to malt affects the dryness. for a 9% ABV beer, 2 pounds of sugar gives a dryer result than 1 pound.
It helps let the fermentation begin with only malt in the wort. Then, when the bubbles in the airlock begin to slow, heat and dissolve 1-2 lbs of sugar in two cups of water. Cool it down and add it to the fermenter. The yeast will kick back into overdrive and fully dry out the beer.