I know there were some questions about grocery stores a little while ago, and I thought I would fill you in on my experience.
As said previously, there is the one called "the grocery store" which is more or less in the middle of Whistler village (I personally didn't go here so can't say much).
There is also marketplace IGA which is in Whistler village north (in the market pavillion). For people staying right at the base of the mountain, it's about a 15 minute walk since it's at the other end. I was at the Delta, right between Whistler village and Whistler village North, and it was a 5 minute walk for me.
I found the marketplace IGA had very good selection and good prices. There are lots of ready made meal options (i.e. bbq chickens and loads of side dishes, and much more). A lot of the stuff is the same price you'd pay at home, some is just a little bit more. I was in Whistler for 4 days and went back to the marketplace IGA 4 times to pick up snacks and meal items, because it was so convenient and good prices.
I had tried stopping at the save-on-foods in Squamish, but it was closed (was after 9pm). In retrospect I wouldn't bother stopping there again next time since the IGA in Whistler had such good selection and decent prices.