There are good reasons to vote early and often, but accepting answers is somewhat different from voting. For the first few (two?) days of each private meta the focus is on asking questions, not answering. The experts who would be able to give detailed, well researched answers will probably do it after emptying their asking buffer and giving their first few loads of votes. In the end of the private beta, we need examples of excellent questions with excellent answers accepted.
When you accept an answer early, you signal that you are satisfied, discouraging those later well-researched answer, and also other good answers adding something new. Having more answers per question is good in general (though there are cases where it is counterproductive). Also, when some better answer arrives, you should re-accept the answer, so that the best answer is accepted at the launch of public beta.