It's good advice. One thing that I notice anywhere but certainly on Flickr which is where I hang out a bit, photographically speaking, is that there's so often extraneous material in a photo. For example, someone might post a photo in which a blank and uninteresting sky makes up 50% of the image.
It's great to get it right in camera, but sometimes you are at the limit of your camera's zoom or you just didn't happen to see the best composition when you were composing the photo. But you can always crop in post, whatever program you use, and it doesn't have to be a fancy program like Photoshop. Any photo program will support cropping.
Yes, you are losing pixels, but many images will still be big enough if you want to print them, and let's face it, most images don't get printed anyway, and the image will still be fine and dandy to show online.
Having said that, sometimes you want context, so tighter isn't always preferable. For example: if you are taking a portrait of a person, you may want to show where they are. If someone is sitting on a bench under a tree or by a river, it may (or may not) be important to include the tree or the river. In this photo, the autumn trail was a very important part of the image, so I wouldn't want to crop it. Context can be important.
However, the exceptions can also prove the rule. Below is an original photo followed by a cropped version. They both have merit, but I decided that I liked the tight crop better in this case as I find that there is less to pull my eyes away from the subject.