"My plans are always practical, it's just the laws of reality that get in my way."
I think that under certain circumstances, and only then, it is useful to blockade off a single planet. Mind you, single planet.
As I'm still down, playing on normal, I won't assume that I have expert knowledge of the strategies, but my approach to war is more Guderian style ("Nicht kleckern, klotzen!"), gather a mass of reasonably agile ships and steamroll over a chosen enemy sector. Then send in transports while your main force already has moved on, cutting of reinforcements. Since planets can only build one ship at a time, small numbers by each planet can maintain space superiority.
Make peace once you don't have any transports left in your first wave and consolidate.
I would only try a two-pronged attack when I have two industrial centres (several high-output planets) that can build up a sizeable fleet each.
But then again, I'm still trying to get up to 'tough' so I may be wrong.