If I build a spiral wrap on the straightest axis, which way should the slight curve be positioned, up or down?

Up. The weight of the components will tend to bring it back to straight, at least somewhat. Plus, you have the maximum deadlift capacity available in that position.

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Robert,
My Kirkman makes a good point.
However, if I ever have a rod with a bend in the tip - I always have it pointed down. The primary reason that I have it pointed down, is that I have found that it is less visible and obvious to the casual observer.

But, the custom builder always has his choice on how the rod is to be built.

Take care

i let the weight of the guides straighten the blank, so the tip would be facing up with the guides underneath, same as for a spinning rod. With a conventional casting outfit the where guides are on top, the bend still pointing up. So I agree with Tom, let the guides straighten the blank. There is absolutely no ill effects doing it this way, and it works.
Norm

In addition to max dead-lift, I also think that because it’s the stiffest axis, it’d provide max sensitivity as well…..at least for certain retrieves. Whether the difference is discernable would be debatable.

I have been building mine tip up with blanks with barely noticed curve in the tip.

I have a " blem" blank with a more obvious curve that the components won't come even close to straighten out so I am using Roger's logic for this scenario.

I'd still go tip up. That's the only way to get maximum deadlift power and maximum speed out of the blank. You can certainly do the opposite, but you will give up a bit of performance. A bit.

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I guess I better attach some guides on before wrapping to see how it looks but since I bought a blank knowing there is a curve performance is certainly most important.

With this particular blank it feels by hand like the soft side is opposite of the stiff spine which is not the norm. Could this be the reason for the more obvious curve?

Thanks for the responses. My first inclination was to put the (very slight) curve up but I thought I read somewhere there was a reason to turn it down. Up it will be.

I agree with Mr. Kirkmans comment about building with the curve up, gives a touch more dead lift.

But, any rod that I have ever built, I have never been concerned with the absolute total dead lift, and I prefer the look of the rod with the tip down, so I will continue to build with the curve down for cosmetic purposes.

Take care