In general they will flip to the correct direction themselves when you fire up the engine but you can do that by hand before installing it. It's always best to eliminate that stress on the few blades that are already curled the wrong way on the cam but if they are properly lubricated, this isn't that big a deal, I just wouldn't rely on them already being lubricated properly enough to eliminate the stress of flipping them in place. The most important thing is what bud mentioned; you don't know how long those few blades resting on the cam have been in the bent position. It's unlikely that they've been cycled periodically since they left the factory so it's best to be sure but you might be able to see through the ports if they've taken a set as you turn the pump by hand. If you take it apart and pull the impeller, just make sure you don't open up a can of worms by wrecking a gasket or nicking the two mating surfaces while trying to get the impeller out. Better yet, remove the cover, and turn the pump without removing the impeller and check that the blades on the cam straighten out when you turn the pump. Then just put the cove back on. Hopefully the gasket will remain intact.
BTW, the blades resting on the cam for extended periods is one of the reasons why I give my engines and genny a quick crank a few times over the winter. I don't like the impellers and other major engine components like springs, etc. resting in the same position for that long. Maybe overkill but it makes ME feel better.