You don't have to reset every single spark phase, but taking 40+ stacks puts huge unnecessary strain on your healers just so you literally get 1 more second of DPS. Not to mention depending on your group's DPS you may even have to wait for a new add to spawn in the first phase making this absolute miniscule gain not worth anything at all.
The hard part isn't even related to the first phase, it's the damage from the exploding sparks and the pulsing AoE during the pillar phase. You might not notice it as a Rogue who I'm hoping is making good use of Feint but the AoE damage is quite large. It's not uncommon for guilds doing this their first time to run into healer mana issues. Hence why taking more damage for the sake of negligible DPS just isn't a good suggestion for a first time group.
Why Rupture if I may ask? Rupture ticks every 2 seconds, so you get one tick off in total. Since you are trying to maximize uptime and damage on the boss why don't you just use those combo points to Envenom Elegon himself? Regardless it's still 1 second of your poison being applied and the application of Rupture. So after that you got 2 seconds to deal 246k damage per second not counting the 1-2 auto-attacks you get off.