The purpose of a straight cut through hair style is to minimise the number of hair 'in contact' with the outside as the rubbing will damage it more. Curly hair being less organised in this 'inside-outside' it allows more flexibility in the shape of your haircut. But making sure that you avoid friction on your hair is still a must and the straight cut on the back is still a must. You can cut your hair when wet to make sure it's as straight as possible or cut it straight considering the natural shape of your hair to make it look more natural, this is up to you!