Twitter was created as a social network, and if that's all you use it for then feel free to Tweet whenever you want. However, if you are trying to use Twitter as a way to promote a product or even yourself then timing is everything. The average life of a Tweet, afterall, is only 4 to 8 minutes. During that time you will want the maximum number of people viewing it.
There are four primary times of the day for your Tweets to be seen. The first is early morning on work days. Sadly for most employers, the first thing most people do when they first get to work is check their personal e-mail an social networks, like Twitter. The next best time to have them read occurs during lunch time. This is a great time to Tweet about lunch specials, or even 1 hour sales.
Sorry employers, again, but the next best time is just as the work day is ending. Most people check their personal messages then knowing they have a commute, dinner, even kid's homework to deal with for hours before they have any "me" time. The final, and perhaps, best time to Tweet for potential profit or viewership is just prior to most people's bedtimes. This is the one time of day they feel the least rushed. A person will gladly give up a little sleep for something that interests them. This is actually the best time for blogs to spark someone's interest.
If that was me, and I actually cared whether people saw what I Tweeted, I would try maximizing the times mentioned above. If you are all about you and don't care, then you wouldn't of read this anyway.
While I have your attention I'm looking for followers, please follow if you saw this. Hope I got it out during the right time!