Cross over does not cure bump steer by any means. A flat drag link in stock config is about the best you'll get for street manners. Cross over is pretty much impossible to get the drag link flat, so by definition, any upward motion in the axle is going to act as though you turned the wheel to the left. More angle means more "turn" for the same compression. A track bar can eliminate that, but it has other implications for articulation and stress loads when used with leafs.
As mentioned, a *slight* downward curve might be acceptable since most movement "at speed" (where bump steer matters) will be up. So if you expect an average compress of say 1-2", you might angle it down by 1/2 - 3/4 inch or so to "split the error". But, that will make things *worse* going over a whoop. Even though axle movement path plays into it and may make some small angle best on the average, unless you have some extremely good engineering skills and modeling software, I would suggest that flat should be your goal...
And bump steer is the tendency of the truck to dart to the left without any steering input when you compress the front suspension (or reverse for drop out going over a "whoop"). It's cause is in the "fixed end" of the drag link (at the frame) forcing the fixed length drag link which forms a radius that the opposite end moves around when the axle moves up and down. If angled down, then the "x" value of the relationship (horizontal distance from DLE to DLE) will increase as the axle moves up, and decrease as it moves down. This results in the turning effect. Of course axles don't move straight up and down, so depending on spring arch shackle angle, suspension design (like links or radius arm) these relationships and rate of change differ from setup to the next...
