I'm a big believer is knowing the rules of correct grammar. You should know what verbs, adjective, adverbs, etc are. You should understand all the correct stuff. But I'm also a big believer in writing however you want.
Now, if you decide to write like this - "hay dude what RU up 2?" - chances are that people aren't going to be so interested in your writing. Which is fine if you are only writing for yourself. But seriously, is it such a big deal if your writing ends in a preposition? Sure, if it's technical writing, say a brochure or manual. But if I'm writing a novel, and want to start a sentence with "but", which I just did, then I will.
I suppose it depends on your audience you're trying to reach. But everyone has a different style. And as much as I love Mary Higgins Clark or Mitch Albom, I have my own style of writing. I'm not looking to make millions from writing - althought that would be nice! I'm writing, because I love to do it. Hopefully enough people will be interested in my stories that I can have some published and make some money from it. But in the end, I write because I love it.