When designing a new hair system, it is important to match the system hair texture to your own natural wave pattern. If you have curly hair, you need to have a curly hair system. If not, there will be a definite line of demarcation and the hair replacement will look phony. If you straighten your hair permanently, then you can get a straight hair system. If you have wavy hair texture and don’t blow dry it straight and your hair system is straight, the “layers” of hair will separate. On a day when the weather is humid, your natural hair will curl up and your straight hair system will look funny! Separation will occur between the processed hair in the hair system and your own growing hair. Not good.
Be consistent with your hair. If you have a wavy hair system with wavy hair, you can straighten both together (assuming the system is made of human hair) and look natural. My point is this: I feel it is best to match the system to your most regular-worn hair style. If you wear straight hair you can curl your hair and the system with a curling iron if you want. But at the onset, match the the system texture with your own growing hair texture to ensure consistency, blending, and to achieve the most natural look possible.