It depends, for me. For example my head-canon is that Rainbow Dash and Applejack and dating each other, because I think its a cute pairing. The brony obsession with Bubblecup (Ditzy Doo) has made me not want a thing to do with toys of her, even though I actually like her and absolutely love the one fanfic I read about her (which is now my head-canon for her). Though other things have also influenced my feelings about the Bubblecup toy (Hasbro's treatment of older fans mainly).
However I don't really collect G4 anymore due to the complete lack of quality control. I plan to buy the white FS Celestia (I'm pretty sure I saw that? If not talking will do I guess) because its actually a correct Celestia finally, and Zecora because I really like her character and the toy is pretty unique and different for the line. I mainly plan to collect the blind bags, and only the ones I really like. I want the more unique ones the most (the alicorns and males), but that's not influenced by character as much as it is finally getting some more difference in blind bags.