The above pictures are of Ranma sneaking up on Akane and surprising her (the first one is actually the setup to his surprising her, results are on the page after it - not shown here). This is a tactic Ranma uses quite often in the manga, on many characters(not just Akane), for varying reasons. Most of the time he does it is just to see their reaction, though he has used it in battle as well. No character can consistently prevent Ranma from sneaking up on them, the picture below is of Ranma sneaking up on Cologne.
As the manga progresses he tends to do this more often. This tactic is not used as often as I feel it should be in Ranma fanfiction. There are several fics I've seen where using this tactic would have been both in character and beneficial for Ranma to use but he did not. Fanfiction Ranma prefers frontal assault far more than his manga counterpart who will often use other means.
Ranma doesn't always stealthily approach or just show up faster than the person can react to, he and most of the cast use the much more common ambushing practice of concealing yourself until the opponent passes by. They use this for various purposes - surprise attack most common, information gathering less common but still done. This has varying degree's of success, for example, Gosunkugi's and Kodachi's attempts are not very successful. In fanfiction it is far more common for villains, rather than the protagonists, to use various ambush techniques, even though it would be in character and extremely useful.