I personally interpret that it was either at the end of the fourth season or in between season four and season five is when Willow finally was confident enough to self identify as a lesbian. I also think that during the time of her getting to know Tara and discovering her feelings for her was when she was in her questioning period, especially once Oz reentered the picture briefly to make her truly examine herself with these conflicted feelings with her previous relationship versus her newfound one. It just seems like a more natural progression of how we got to the fifth season, imho.