Personally, I try to keep the language mild. I feel it's unrealistic to write a character who would absolutely curse and never, ever have them do it. 'He/She swore' works a lot of the time, but not for every situation. Yes, mature people in real life often can articulate something without swearing, but with real, palpable characters, there are moments other than moments of articulate, carefully-constructed expression. People blurt stuff out, say things without thinking, react to things, and yell stuff - they just do. No, you don't need to 'be' your characters to characterize them adeptly, but all you have in a story to convey a character is words. I'm personally of the the opinion that certain words, though they may be harsher, are sometimes necessary. Don't get me wrong - there are some words you'll NEVER see in anything I write - but I don't see what's so horrifically disgusting about the 'd-word,' for instance. Yeah, it can mean to be damned to hell, but it's also an expression of frustration that beats a lot of the crass or vulgar ones by a long shot, and a lot of the characters I create would have no problem using it. I don't like to use it in life - but they're not me. I draw the line at more shocking words so as not to dishonor God or pull focus from the story, but yeah. Those are my feelings on it.