I have seen some comments on the forum in the past concerning how underwraps would effect the action of a rod. Did a seacch and could not find what I was looking for so here are 2 questions. I am talking about bass rods here.
1. Does underwraps effect the action of a rod? (I remember seeing different comments).

2. If you layout a guide wrap that is more than say 4 to 6 threads past each toe of the guide foot, say 5 mm of thread would this have an effect?

3. Does it matter or are we splitting hairs?

Depends on how much hair you have to start with!

If you are building a bass rod for performance overall total weight is the controlling factor that defines that thing called "sensitivity". If you goal is to provide the most effective tool for fishing anything you can do to reduce the overall weight of the rod will improve performance.

Start using a digital scale to weigh things added to a rod blank and you will soon see the importance of weight of all things including thread and finish and it location on the rod blank.

The delivery of a long throw micro eight foot powerful craning rod under 3 ounces allows me to proudly display my bald head with the split hairs -

Gon Fishn

No, it will have no effect on the action of the rod. Action refers to where the rod initially flexes.

However, it will have an effect on the speed of the rod. Speed refers to how quickly the rod reacts and recovers. The more weight you add to the rod, particularly in the upper 1/2 of its length, the more you reduce the efficiency of the rod and slow the reaction and recovery speed of the rod.

On very heavy offshore boat, trolling or surf rods, we might be splitting hairs. But on lighter rods it's actually possible for the human hand to notice the difference in speed and feel between a rod with underwraps and the same rod without underwraps.

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No they won't affect the action but it slow down the recovery of the blank. You have more matter which means more momentum.