I feel if you want to keep a design to yourself you shouldn't post it online, anywhere. Other wise people will use your cakes to draw ideas from.
While I think it is nice to ask, I don't think you have to. It is also nice to credit who/what inspired you, but I don't think it is the end of the world of you don't, as long as you don't play it off like you created it all by yourself.
My Weight Loss Support Group is The Chunky Monkeys!
My Weight Loss Support Group is The Chunky Monkeys!