Woody,
I've done little float fishing for summer-runs, but lots for winters. I think the riffle-loving summer-runs are easier to approach with spinners and flies. Anytime I fish float/jig, I always have the jig a foot or 2 above the bottom. Since summer-runs will often come to the surface for a fly, I bet you don't have to be as close when fishing for them with jigs. You can fish for them more efficiently and confidently if your jig is off the bottom.
As for time of day, I usually do better when the sun's off the water, especially if the river runs south to north. Throwing spinners in the riffles and choppy seams seems to work any time of day in the summer, though. Just go smaller and duller if the sun's bright. I imagine this would apply to jigs too.