The Handling Input Method Visibility page in the Android developer docs has this to say about showing the input method on activity start:
Although Android gives focus to the first text field in your layout when the activity starts, it does not show the input method. This behavior is appropriate because entering text might not be the primary task in the activity. However, if entering text is indeed the primary task (such as in a login screen), then you probably want the input method to appear by default.
These guidelines make it fairly clear that this behaviour doesn't contradict any rules (and it might make it easier for the user to navigate your activity).
That said, it really depends on what you have in your activity - in particular, whether the keyboard obstructs any important information you're trying to give to your users.
As that page says, there are plenty of options you can set in your layout to control the input method and the UI behaviour, but whatever you do, it's worth looking at the accessibility guidelines to make sure your app is accessible.