The ideal zoom level depends on your image and the details you want to emphasize. You should try different zoom levels.
Yes, using zoom sparingly is often more effective. Don't overdo it to avoid distorting or overloading the image.
Images with clear detail and main subject work best when zoomed in. Featured portraits or images are good candidates.
Yes, most image editing programs and applications allow you to adjust the zoom level afterwards.
If you have instazoom into the picture, you can also move it to get the desired section.