Chocolate baking bars vs. chocolate chips?
I would say that if you're melting chocolate, it's fine to substitute chocolate bars - maybe even preferred. Chocolate chips are a much lower quality chocolate than real chocolate bars. (I've read that chocolate chips contain less cocoa butter, so that they hold their shape better in cookies - which also makes them harder to melt.) In general, I've found that when baking with chocolate, the higher the quality of the chocolate, the better the end product.