It's one thing to buy yarn that you feel will knit up beautifully as well as feel good on the skin; however, buying yarn that is ridiculously expensive is never a good way to build a stash. I go by three rules:
Texture/Appearance: Will this color or texture look good once it's knitted up? Is it comfortable against my skin? Would it irritate my intended recipient's skin?
Practicality: If my forehead sweats, will the edge of this hat turn into a felted, puckered mess? Can I machine wash it? Can I ever wash it?
Affordability: Not all affordable yarns are crap. Some actually wear better than the expensive yarns at your LYS.
I have a sizable stash. It's actually scary beyond all reason because I have no idea when I'll ever use half of my yarn. I have an assortment of synthetic yarns to novelty as well as some very luxurious wool and silk yarns. Some yarn must-haves for me?
~Wool. Lots and lots of wool. I love Patons Classic Merino and Lion Wool and have several skeins in my stash in different colors.
~Acrylic, preferably Caron Simply Soft. This is an excellent choice for people who are allergic to animal fibers.
~100% Cotton. Sugar 'n Cream, Sinfonia and Lion Bran Microspun can all be found in my stash. I typically use the Sinfonia as a waste yarn because it is mercerized and very slippery.
~A few hanks of the most luxurious yarns I could get my hands on. I'm saving them for special projects.
It's all about balance. Yes, expensive yarns are nice but they're not a necessity and they're not always practical either. Over time you'll learn what works best and then you can build your stash from there.