Personally, I don't feel it matters. You dress up as a cosplay character to show your love for that character and to have fun, not to show off your uber sewing skills. At least that's how I feel, anyway.
I can see both points, though. Putting your own outfit together makes your outfit more "personal" and you "know it better". But it doesn't necessarily show that you're more dedicated to a character or anime series than someone else, nor shows that you love your character more than someone who bought their costume does.
Whether you buy or make your costume it doesn't show that you love your character or are dedicated to that character any more than anyone else is. Isn't cosplaying all just about having fun, anyway? Why bash someone because they didn't have the time to sew, can't sew, didn't want to sew, were unable to sew, or found it cheaper to buy the outfit already made? I think that's disrespectful. After all, they're still there in costume, cospalying a character they love, and are having fun. Isn't that was cosplay is all about?
I've done both, though. I've made a costume (though it wasn't an anime cosplay, it was Star Wars, and I'm working on another), and I've also bought a costume from a costumer on Ebay.