I actually do schedule some Tweets.
When I know I'm going to be out of the office for a couple of hours I'll schedule a few tweets to be posted for when I'm away, and also sometimes before I go to bed I schedule Tweets so that people in other TimeZones who aren't awake when I'm actually on Twitter don't miss out on the opportunity of hiring me as their VA

I like to keep my name at the top of peoples minds so when they think that they may need a VA, they think of me first (hopefully), and if at that moment they are on Twitter and they see one of my Tweets pop up, all the better for me.
The Tweets I do schedule though are usually VA tips, and also links to my latest blog post.
When I am in the office I converse a lot on Twitter and post other peoples blog posts, articles of interest etc that come up at the time.
So I think there is nothing wrong with Scheduled Tweets as long as you do it tactfully and reply to people's @mentions and RT's as soon as you get a chance.