Here's one that people often forget: When it comes to RPing as certain characters, your spelling and punctuation count. DEFINITELY. If you don't make proper sentences, you lose all affect your character may have on another person, not to mention you totally lose the character in general. To other people, misspellings and grammar mistakes among certain RP roles are just disgraceful. To other people, you look incompetent, ignorant, and overall inaccurate. I'm not talking about small mistakes like not putting a ' (apostrophe) when you're spelling "don't". I'm saying you shouldn't talk like you never went to school.
When you present your character, think about how it would fit with their personality. Your avatar (if there is one needed for your RP) should reflect how the character would want to present themselves. A character like Itachi would never put a "cute" picture up (i.e., a picture of him as a neko) but Naruto would. The reason is not "masculinity": it is simply the character's sense of humor.
ANYWAY, BACK TO THE SPELLING AND STUFF.
If you are playing someone like Kaname Kuran from Vampire Knight or Itachi Uchiha from Naruto, your grammar should probably be almost perfect.
Those characters have a "dignified" aura about them. You may TOTALLY lose their "voice" if you have EPIC FAIL English. So....aim more for:
Itachi: It was a pleasure meeting you, Hime-san.
Itachi: taht wasn't what i was trying to say.
THE EFFECT IS GOOOOONE.
For certain characters it's OKAY. Naruto and Luffy, for example, probably don't really CARE about proper English/Japanese as characters! Still, it's best to keep some sort of dignity.
Naruto: EXACTLY! I'M NOT TOTALLY ILLITERATE- TTEBAYO!
Naruto: But I HATE EDITING STUFF soo...erm. I'll just leave it. ^w^
Itachi: Nothing, Naruto-kun. Nothing at all.
Hope this helped!