Unless you are trying to stem a crisis requiring an immediate release, I have found 7:30am-8:00am in your “target timezone” to be the most effective. Note your “target timezone” may be different than the location of your business. For example, if your technology business is in Dallas but you want to reach out to the San Francisco tech media, a 7:30am PST release would be advisable.
I would be interested what others have found to be most effective in terms of timing.