Fish are funny. It seems, for the most part, that they want the lure further from the boat when you're fishing shallow. BUT, you'll also pick up some fish right in your prop wash. They certainly don't follow any rules.
Personally, I wouldn't bother with flashers until a thermocline sets up and you're fishing deeper in the water column. Early season, I'd concentrate on getting my lures away from the boat... both out to the side (with planer boards) and back (by using long lines).
But that's just my preference.
I have had decent luck this spring using flashers with the lure 5 to 15 feet back and 10-20 feet down. I also use Roemer releases which put the lure about 3 feet above the ball/flashers, for whatever thats worth. I have also had trips where the downriggers don't produce and flatlining leadcore does. I change something (lure type, lure color, depth, etc.) every 20-30 min. if there's been no action. Sometimes you just wind up burning gas$ and rinsing your tackle. Keep at it.