I was talking to an Akane fan earlier and they were talking about how Ranma deserves to be hit by Akane because he always insults everyone. This is not true; Ranma does not insult everyone, even if that was true it is not a good reason to hit someone.
When Ranma first meets people he is nice and doesn't insult them until after they have done something to upset him. This is shown many times in the manga. When he first came to the Tendo's he was polite and not insulting until after the Tendo's insulted him repeatedly. He was polite to both Ukyo and Ryoga (contrary to some peoples way of thinking Ranma would rather talk than fight), etc. If your polite to Ranma he'll be polite to you. He is always polite to Kasumi, even uses the -san honorific for her. He is even polite to Happosai when Happosai hasn't done anything to annoy Ranma for awhile.
Ranma accidentally insults people sometimes because of his inadequate social skills (doesn't think about ramifications of what he says and treats girls like they were guys -which isn't that uncommon for people who went to all boy schools) and his upbringing. An example of accidental insult is: During an early volume of the manga Ranma wears Akane's clothes because his clothes are in the wash and Kasumi and Nabiki pressure him into it. He is asked how the clothes fit and replied honestly that top was too tight bottom was too loose, inadvertently insulting Akane. No insult was intended.
Sometimes he insults people because they insult him (ok often). An example of this is most of Ranma's dialog with Pantyhose Taro.
Ranma also uses insults as a means of manipulating people or to serve a purpose. He uses these type of insults to spur someone on, anger them in battle, trick them to do something/not do something, and to avoid things (like eating Akane's cooking).
Some of Ranma's insults may have been meant as compliments but were worded wrongly or not taken in the manner Ranma meant them. Akari is the person who did this the most with her comparing Ryoga to a pig; they were meant to be compliments but were taken as insults. I think Ranma does this as well but can not think of a specific instance where he did so off the top of my head.
Contrary to what some Akane lovers say Ranma was not the one who insulted Akane first. She insulted him multiple times before he insulted her. See the first volume where she insulted his gender and curse when he makes a comment in retaliation to her insults she hit him with a table while his back was turned. Considering the way the Tendo's and Akane were treating him (basically arguing about not wanting him, treating him like an object, and insulting him) his comment was fully warranted. Ranma really doesn't insult Akane much more often than she does him (if he in fact actually does do so more). I'm fairly sure there are volumes in which Akane insults (either purposely or indirectly) Ranma more often than he does her. I've had people argue with me that Ranma's insults are worse than Akane's this is not true. Akane's insults to Ranma are as bad as or even worse on occasion than the insults Ranma uses on her. For example, she has accused him of trying to rape Miss Hinako, in volume 8(VIZ) she said that his body has always been sick and twisted, told him to stay a girl and marry a guy, etc.
Ranma does not always insult her when given a reason to and has attempted to not insult her on a several occasions. For instance some people say that Ranma always insults her cooking, sewing, etc. In the manga, when she first cooked for him he took one bite and tried to just continue training instead of insulting it (she then tried her own food, said it was bad, and threw a rock at him). As for her sewing he did not purposely insult it, ever. Two instances exist in the manga, one he did not know she was around and said "how can something sewn that looks like it was sewn by a third grader be so hard to pull off!?" and the second time he said "that's a rabbit I thought it was a jellyfish" which isn't really an insult particularly since it looked like a jellyfish. Since I'm trying to be fair Akane does not insult Ranma every chance she gets either and she has on occasion not hit/insulted Ranma when, in my opinion, it would have been excusable for her to do so.
In a related thing, since cuss words are most often insults, In the original Japanese and all the English fan translated versions of the manga Ranma, and some of the other cast members, use swear words (or extremely rude language that if properly translated would be equivalent to cuss words) .