There is NOT some sort of hidden bonus for using materials from the same tier, if that's what you're asking. The stats you see when crafting are the stats the weapon has, and that's it.
Here is a way to visualize it...
Weapon A - tier 3, tier 1, tier 1
Weapon B - tier 3, tier 3, tier 3
3 + 1 + 1 = 5. To average this out you divide by 3 for the amount of slots; 5/3 = 1.6
3 + 3 + 3 = 9. 9/3 = 3
So, you can see weapon A is still an inferior product either way.
Now, another discussion you can have is whether to use higher end mats on a low quality weapon. Let's say you REALLY like the Longsword of the Dragon and want to craft the T2 version as soon as you arrive in Skyhold. If you have T3 materials, you can use them to craft the T2 sword -- or any lower weapon -- to extend the life it would normally have as a typical T2 weapon with T2 materials.
Some people would say this is a waste, I'm sure. But it's your game, and you can use the mats however you want. This is especially true if you're on PC and can use Cheat Engine or mods to give yourself more gold, mats, etc. It's also dependent on the difficulty you play on. Someone who plays on Casual (like myself) can be less concerned about the stats of a weapon and focus primarily on looks, whereas it's less practical on Nightmare.