If you are providing content for compressed formats like YouTube, iTunes and so on then we do recommend you check the discussions from iZotope of the best way to prepare your content for these and similar platforms. They have included audio examples showing what can happen to audio when it is converted to these compressed formats as well as tips and suggestions on how to master for iTunes, SoundCloud & YouTube.
More and more of the content we produce, whether it is music, radio, TV, or video is going be delivered to the end user using a compressed audio format. For music the most common compression formats are MP3 and AAC. Wether you like it or not, these two codecs represent a compromise between file size/convenience and audio quality and they are likely to be around for some time, particularly with the popularity of iTunes, YouTube, SoundCloud, and streaming services such as Spotify and Pandora. When digital audio is compressed for delivery on these platforms, it is inevitable that there will be artifacts and audio quality tradeoffs.