Can I run THHN wires through an attic without conduit?
Run it in conduit the whole way. 

Attics are susceptible to rodents, especially the cute ones with fluffy tails. They chew on wires. Conduit is not that expensive, and can save a whole world of trouble.

I have seen way too much romex chewed down to the copper to trust romex where rodents could be - which is going to cause me some hassle and expense. But I hate burning buildings, especially if they are mine.