I agree with Sivaprasad that it matters only when you are writing/typing. But when you are typing, it is quite important to leave a blank space (in the US, we call this simply a space) after every comma and period and also, of course, after each question mark, exclamation point, colon, or semi-colon. I cannot think of any punctuation mark that is an exception, although someone may correct me. I am certain about the need for a space after the items I named. Good question! :)