Should I unscrew the bottom of the Difrax S-baby bottle a bit when feeding?

You don't have to hear air escape from the bottle or unscrew the bottom a little while feeding. 
If a vacuum is caused in the teat, this means the silicon valve in the bottom is not functioning properly. 
Some children have such suction strength that they manage to cause a vacuum in the teat after all. In such a case unscrewing the bottom just a bit can help, as more air can escape from the bottle.