The two alternatives in my question of course lead to very different results. If you use \newline, then your paragraph will not begin indented. Also the spacing between the paragraphs differs.
I am asking this because this usage of \newline catches my eye in lots of documents.
In addition to what Joseph said, No, it is not good style. If you want paragraphs without leading indentation, use the parskip package.
You are better off in LaTeX marking up what things mean and letting the engine decide how they ought to look. So, in general, when you are tempted to stick in some fine-grained layout command, you ought have a second thought. While thinking, see if there is a package that will provide the layout you are after globally. (In the case at hand, once you've sprinkled \newline across your document, it will be more difficult to change the appearance than it would be to comment out the \usepackage{parskip} line.)
\newline does not start a new paragraph, it just forces a new line. So the result is very different: use two blank lines (or \par) for a paragraph, and \newline (or \\) when you want a new line. The later is rare but of course does happen (particularly in titles and so forth).
\noindent. – Caramdir Aug 04 '10 at 17:37\noindenttoo often in your document, though, it's a sign you're probably doing something the wrong way. – ShreevatsaR Aug 04 '10 at 18:44