It looks pretty accurate to me.
1am PST is about the time ZDNet editor Larry Dignan wakes up. He follows me on Twitter, so maybe he would retweet or promote a blog post I tweeted about at that hour.
5am PST is when most of the east coast is waking up. 3pm PST is when most people are travelling home on the east coast (skimming Twitter on their mobile devices). On the west coast, 3pm is when most people stop working for the day and start reading Twitter. And 5pm PST is a perfect time for me to tweet because most of the east coast is checking Twitter for the last time for the day, and my west coast friends are just leaving work.
You can also schedule tweets during those hot timeframes:
This is a pretty cool app, and I think I'll give it a try next time I write an interesting blog post that I want a lot of my followers to read and repost.