Why do cats stick their tongue out?

Is it ever a warning sign of a problem?

I remember once when we took at cat in a car when we moved for 280 mile it kept meowing and stuck it's tongue out.

sometimes it does it now but usually between cleaning itself

If it happens during a car ride it's a good warning that your cat is feeling car sick. The tongue comes out right before they vomit.
It can also be out if the cat is panting, if there is a mouth/tooth problem, or just one of those cats that likes to hold it's tongue between it's teeth.

My kitty just likes to stick her tongue out,

My cat Puffy gets relaxed, and his tongue comes out, further, further... I never realized how long it is. Dear Husband calls him Lizard Tongue because he can drink almost all the water from a mug. In addition to their regular water bowl, we put a mug out under a chair in the living room for them, they seem to love drinking out of mugs as we do and keeps their water consumption up.

One time Puffy got so involved licking his chops after a treat his tongue just kept going, like a broken windowshade, and we had to reset him like a computer. We press down on the top of his head and he gets over his confusion because he has something new to focus on.

If neither Puffy, nor you, nor your husband, has seen "Earth Girls are Easy," you need to rent it. Watch the scene in which a young Jim Carrey "drinks" from a glass. lol!