Using the cry it out method, or extinction method, during sleep regression is really a personal choice. If your primary sleep training method before the 18-month regression appeared was the cry it out method, you could continue using this as it will keep your toddler’s bedtime routine consistent. On the other hand, they may resist.
Most parents find success during regressions by offering some sort of comfort to the child in order to get them to fall asleep independently again. Though this tends to go against the principles of the extinction method, it can be a momentary, useful tool as you work through the issues that temporarily disrupt their sleep cycles.