I would strongly advise against resetting stacks in the final phase. As soon as the platform comes up, get straight in there, BL and blow some healing cooldowns. Finish any adds, then just full nuke on the boss. Once the first tank is out of cooldowns, swap over. Collect loot.
Moving off the platform in the final phase does reset your stacks, but it also give you a huge damage hit, as well as the danger of being away from the healers and raid. Not worth it at all.
I reckon 4-4 is the minimum number of spark kills you should be aiming for. Ideally, more like 5-4. We do 6-6 because it's fun to watch a boss die really fast, but it certainly isn't necessary.