Always take pictures with the camera app first
Taking pictures with the camera app will allow you get that perfect shot as well as allow you to take advantage of the iPhone’s built in HDR function. The HDR function can be particularly useful for shots of still objects or landscapes outdoors in sunny conditions or high-contrast lighting situations. Also, using your iPhone camera you can save the original without filters for your files or for editing on your computer (or using one of the many iPhone editing apps). And finally, your iPhone camera app has some useful features like a focus and exposure lock mode.
But, remember that your iPhone camera doesn’t shoot in the 1:1 ratio used by Instagram so you will have to crop your images (keep that in mind when framing, sometimes just taking a step back can give the perspective needed for when you crop the photo).