Full text justification does not work well on web pages for two main reasons:
the way we actually read text when we're viewing it on screen
the way browsers handle text justification
Text is more difficult to read on screen than it is in hard print because of differences in monitor resolution. Furthermore, even though it is not obvious to most internet users, the image on the screen is not totally static; in fact, it is constantly being refreshed. Different types of formatting that include inconsistent spacing may make it difficult to read words, phrases and paragraphs.
Internet browsers such as Firefox, Internet Explorer, Safari are not very good at handling justification and displaying justified text (and each browser’s display may differ), and your website visitor is likely to be presented with text and spaces between words which varies a great deal. In printed text, justification tends to result in a more subtle variation in spacing. The extreme variation in the spacing online makes the text more difficult to read - instead of the eye being able to move smoothly from one word to the next, it has to move in "fits and starts," searching for and jumping to the start of each word.
You would be hard pressed to find a major website that utilizes full text justification because of the usability issues it raises. The New York Times, CNN and the website for the Supreme Court of the United States are perfect examples. So while the edges of your text blocks may not be perfectly neat, avoiding justification will ensure that all website visitors able to read and comprehend the information on your site.