It is polite to take what is offered. It is also polite to decline things you don't like with a "No, thank you".
I think the proper thing to do is if you're unsure of whether you will like it is to take a very small bite, because if you do not like it, you are not wasting it for the people who do, and if you do like it, you can easily take a larger second helping. But if you know you have strong food aversions or restrictions, then perhaps the most polite thing when confronted with a mystery food item is to say "I'm not familiar with this dish - what's in it?", and then decide whether to take a courtesy bite based on that.
I try to take courtesy bites, but I have some very strong food aversions (fish, black olives), some mild food allergies (peppers, melons), and one possibly deadly seasoning allergy (lavender), and I don't tolerate spicy food well. So when presented with something I've never seen before, it makes sense to me to ask a few questions about the preparation and flavor. I think it remains polite as long as I don't suggest the food is somehow disgusting for daring to be something I don't like, can't tolerate, or am allergic to.