You can write in any manner you choose... just remember that it gives people an impression of you similar to the neatness of your dress and the cleanliness of your home.

If you have sloppy grammar, excessive typos, poorly-constructed arguments, or that horrible number thing -l1k3 th15- people will judge the CONTENT of your posts as well and may assume that you are just generally sloppy in other aspects of your life.

Good writing skills are something in which you should take a lot of pride. If you know HOW to write, why pretend you don't?