If you keep your house between 65-75F then location isn't as big of deal. Drafts aren't going to chill them so much if it's at least 65 inside and sunlight isn't going to cook them if it's under 75. You still don't want a really cold draft say a leaky window in the middle of an Iowa winter which could possibly dump -10F air on them. You also don't want the whole cage in direct sunlight at any time but sunlight from a curtained window, on part of the cage during part of the day, or having them in a really bright room is not going to cause problems provided it's not already 80F inside. If they are away from drafts and sunlight they tolerate 60F to 80F temperatures pretty well. Guinea pigs handle cold temps better than hot ones and at 80F they can get heat stroke if things go just wrong. In hot weather make sure they are not getting too much sunlight, blow a fan on or near the cage, make sure their water bottles are full, you can set flooring tiles in part of the cage to provide a slightly cooler spot to lay, and they enjoy frozen water bottles added to the cage to lay near and chew on or lick.
