Do you use two shades?

Do you use one shade of foundation in the winter and then a slightly darker shade in the summer?

That's Normal

I heard of one person who actually uses a lighter shade in summer. Basically, if your skin tone changes then your foundation should change. Unless, you are using it as a concealer or corrector, foundation that doesn't match your skin won't be flattering (even then the shade difference shouldn't be too extreme). 

Do you need to change foundations? Well, that is your judgement call. If you think that your shade is perfect year round, then great. If you want to switch it up, go ahead. Some people test foundation on the wrist, I read that it should be tested on the jaw line. Check it in different types of lighting-I once used a color that was perfectly fine in the house, but in the car I could tell it was too dark. 

I had a hard time between two shades of the Mary Kay Luminous Liquid foundation so I ended up putting half on one side of my face and half on the other and comparing. I found that really helpful but its not ideal to do in a store. That's the fun thing about buying from a Mary Kay consultant is that you really get to apply the makeup and create different looks before deciding what to get.