Why would you want to give them a heads up if you don't have to? Just curious. If I had the option to A. Position my troops and then attack or B. Let them know I am going to declare war on them, giving them time to prepare, I would choose A.
What *would* be cool (and I don't know if they do this.. I am getting the game later today, but I am assuming they do not do this), would be that if they saw you prepping for war.. i.e. your ships were surronding their planets, hanging out on their borders, etc.. that they would realize this and take action before you attack. Either form an alliance with another race.. offer solutions.. attack you first stuff like that.
Do they in any way do this? Just curious. If I saw a human player surrounding my territory, I think I would get a clue what was going on, and react.
fleabitfox