You can probably get away with just straightfletching if, as you say, the arrows are spined very very close and the bow istuned just so so. The reasoning behind shooting offset or helical is that not everybody achieves that perfect tune or even know there is such a thing as tuning, A lot of people are still unaware that the broadheads must be aligned as perfectly as possible with the shaft. Not everybody works on their form till it's nearly perfect.
There is a point of no return when shooting helical, though. Get too much drag on the back end and groups can actually get larger at longer distances. There's always some balancing act.
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