Most, but not all cats will be a little protective of their soft belly, so prepare yourself for a defensive scratch if you decide to give your cat a tickle.
A tabby cat really enjoying its chin being tickeled
Some cats will come up to you full of love and affection, roll over and stretch out, but when you extend your hand to give them a belly tickle they suddenly seem to turn feral in a biting and clawing frenzy. This is a defensive reaction; the cat’s belly is probably their most vulnerable area to predators. In the wild, a cat who suffers injury to their belly could very easily be killed if any of their vital organs are damaged. Although you are not a threat to your pet, their instincts kick in and they feel the need to defend.
If your cat is rolling over and exposing her belly then this can be taken as a sign of comfort and security. It mean that she trusts you and doesn’t feel threatened, but watch out because her attitude could change at any moment. Unless you know for sure, it's always better to stay on the side of caution.