You should always work with what you love and can afford. If craft store yarn works for you then use it. If you want something more expensive then use it. Clearly don't go broke to knit but knit with what makes you happy. And watch for sales. Some stores have REALLY great sales and someone almost always posts about them here. I have gotten some really awesome stuff for half price or less in some of these sales. Makes it even more delightful to knit with it, i think.