The hashtag is a tool to link your statement to other similar tweets. As a business, it's a good idea to use hashtags to link new tweets with old tweets on the same topic, like #SocialMedia. Hashtag words are automatically linked as a keyword search in Twitter.
If you already used the phrase in your tweet, it's generally considered bad form to repeat the phrase with a hashtag. You could, however, add a hashtag in front of the phrase within your tweet. Hashtags do not have to always appear at the end.
If you have something to say on a trending topic, be sure to include the hashtag. However, avoid commenting simply because it's a trending topic.
Finally, don't hesitate to include hashtags on some of your other social platforms since savvy users may paste the tag into Twitter and look up your old tweets. #TwiiterTip