Astah has several alignment features to align UML model elements nicely with easy step.
But still sometimes you may want to move model elements individually just to place them in a right place. In that case, hold [Command] key down when you move them, so that they will move 1 pixel at a time.
Also you can show Grid on the diagram from [Tool] – [System Properties] setting. See detail (TIPS: Show Grid in diagram)