But we already knew they had feelings for one another. Just look at their child hood, and the promise. And then before Cloud falls into Aerith's church, Tifa says there's still so much she has to tell him, and he says "I know...". Then we see Cloud dressing up like a woman to save her. The entire time during the whole Don Corneo thing, Cloud's focus was all on Tifa. It was like that a good chunk of the game, in my opinion. So I could already see the feelings that were there. What the LS scene did was show they had a bond stronger than just a crush. Tifa knew who the real Cloud was, put him together piece by piece, was one of the center focuses of his mind, and was the only one who could help him. At the very end, Cloud even says "Tifa, let's go home." I always took this line as being very special. Then during the Highwind scene, they just seem to seal the deal. Cloud even says nothing has changed from when they were children, that they have no where else to go now, and encourages Tifa by saying that everything will be alright.
CoT, Cloud says he wants to start a new life with Tifa, and will be okay as long as she's by his side. He even says he'll be around to remind her of herself, when she's feeling down. So that's even furthering their relationship.
AC/ACC they are already in a solid relationship, Cloud is just afraid of hurting his family and feeling worthless. Trying to find a cure for Denzel, and being afraid of losing his happiness. This relationship is NOT still developing into being actually official. It already appears to be very official to me, aka canon. I don't get why people think that, no offense. Think what you wish and all, but I think they were already in a relationship before AC/ACC, and even before CoT.