Should you rotate your garden in a different variety every year?

Crop rotation is very necessary every year.

You should rotate your garden every year to replenish the nutrients that a particular crop needs and the next does not. I keep my garden on a 3 year rotation cycle and do not have nutrient deficiencies. This allows the soil to fully replenish before the original crop is replanted.

Here is an article to help you rotate crops:

http://www.purposefullysimple.com/crop-rotation/

Yes, don't plant tomato's and potatoes together

Absolutely!!

Having a large garden, I keep a notebook on the "happenings" in it. One of the things I have, is a rotation map, that lets me know where things have been planted the year before.

Yes, a 3 year rotation schedule is a good thing.