Jazneo, I think there was some miscommunication in my original post. I never said tipping wouldn't unlock anything, in fact, I said the reverse. Specifically, I said I don't think it has any baring on getting a shiny, but that it DOES effect the invisible "style" stat. According to the page I found from a guide online, the higher your style meter is, the more things you unlock in the game. This list had the clothing shop at Lumiose low on the style meter list (about 3rd unlocked). I remember unlocking interactions with some special PCs also has to do with how much style you have when you pass by that area of the game.
I also remember, at the top of the meter was being able to purchase an end-game item for only 10k where as those with minimal style must pay 1 million for it.
My theory then, is the more you tip, the more style points you're awarded for the action upon completion. The more style points you earn, the more quickly you unlock things based on style.
In short, I'm saying high tips probably pay off by increasing your style points more than a lower tip would for the same interaction (unlocking things a bit earlier than you would have otherwise).
TLDR: Bigger tip = More style points to unlock multiple things in game. :)