It is worth separating the logic from the epistemology. Let's start with the logic.
A (first order) theory is a set of sentences. Usually we are interested in deductive systems, so we require a theory to be closed under the relation of provability. A theory T is axiomatizable if there exists a subset of T, the axiom set, such that all of the sentences in T are provable from that axiom set. Not all theories are axiomatizable, and in general there may be many different ways to axiomatize a single theory. As far as the logic goes, there need be nothing special about the members of the axiom set. They just denote one way of representing the content of the theory. Usually we eliminate redundancy from an axiom set by not including sentences that can be proved from the other sentences within it.
As to the epistemology: a theory often has an intended interpretation. We usually want the sentences in T to be statements that are true about some domain. It is not essential that the sentences are true: they could be just uninterpreted formulas with no truth value, or they could even be false under some interpretation. The theory won't explode unless it is inconsistent. But if we do want the sentences to be true relative to some interpretation, we then have reason to choose the members of the axiom set in such a way that they represent the sentences in T of which we are most certain. The epistemological motivation is to represent the theory as flowing from the axioms; the axioms provide a warrant or a justification for accepting the theory as a whole.
In general then, axioms of a theory do not need to be tautologies, and indeed do not even need to be true. But there is an important rider here. Because logicians have tidy minds, we often wish not only to axiomatize our theories but to axiomatize our logic itself. This means expressing everything that can be proved within some logic L as the consequence of some finite set of axioms and rules. When we do this, the sentences that can be proved within our logic are "logical truths". Depending on which authors you read, "logical truth" might be a synonym for "tautology", though I prefer the usage that "tautology" is reserved for logical truths of the propositional calculus. Either way, we can say of a logic L that all of its theorems, including its axioms, are logical truths, or if you wish to be more cautious, that they are true-in-L.
So, if you are talking about the axioms of a logic, as opposed to a theory, then the axioms are logical truths.