TIM, a widely accepted view (folklore ?) of the steady wind additive is to accommodate wind-shear (decrease) due to altitude gradient. However, there may be wide variations in this (as with gust factors) amongst manufacturers and operators depending on aircraft type and operational situations.
The headwind component is normally considered, but the side-force component, especially of gusts should not be discounted when considering maximum crosswind recommendations (manufacturer/operators).
I would add that it is very important to adjust the landing distance required for any speed additive which might be retained until the threshold, and also consider that any expected decrease in the additive might not occur.