More in response to Kalosyni's original question, I think Epicurus' view of 'reason' is complicated from our point of view by his eccentric approach to vocabulary as described in the Letter to Herodotus:
QuoteFirst of all, Herodotus, we must grasp the ideas attached to words, in order that we may be able to refer to them and so to judge the inferences of opinion or problems of investigation or reflection, so that we may not either leave everything uncertain and go on explaining to infinity or use words devoid of meaning.
[38] For this purpose it is essential that the first mental image associated with each word should be regarded, and that there should be no need of explanation, if we are really to have a standard to which to refer a problem of investigation or reflection or a mental inference. And besides we must keep all our investigations in accord with our sensations, and in particular with the immediate apprehensions whether of the mind or of any one of the instruments of judgment, and likewise in accord with the feelings existing in us, in order that we may have indications whereby we may judge both the problem of sense perception and the unseen.
And we can see this at work in the distinctly Lucretian phrase vera ratio, true reason or true philosophy.
Epicurus rejected reason as a criterion of epistemology, he rejected dialectic as a method of inquiry, and he was suspicious of the cult of formal logic. But in Lucretian terms, true reason is synonymous with Epicurean philosophy, and for Epicurus the outward expression of this true reason is the practical wisdom of φρόνησις, phronesis. The fruits of phronesis, in turn, are good choices and avoidances. This is fundamentally reason in service of the blessed life of pleasure (which again is to be considered according to his eccentric approach to vocabulary).
So far I've discussed the Epicurean view of reason as it relates to Canonics (where DeWitt says it has been "dethroned") and to Ethics, where it is part of practical philosophy. Reason obviously has a place also in the Physics, but I'll have to return to that later.