Ther is no hard and fast rule as to when to consider Clad into design (Strength) Considerations. Generally this is guided by the design Philosophy followed by the designer or the Purchaser or his process licensor or his consultant.
It is common practice to take clad metal for the purpose of Corrosion resistance only and not consider the same for the purpose of Strength Calculations.In which case the corrosion allowance is considered as "0".
Imagine the following situation. If you have considered clad metal in your strength consideration then the required thickness would come down. Then during operation (By unknown chance) if the clad thickness reduces the stability of the equipment would be in danger. These kind of fears and fear of manufacturing quality problems drives the designers to make such choice.
But again there are always exceptional cases where there is still grey area. Take the case of Tube to Tube sheet joint in an exchanger, where the designer has choosen strength weld. In which case the clad metal (Although assumed by the designer as providing only corrosion resistance purpose) also contributes to the strength!!
