How to open with Baratheon at Game of Thrones Board Game?
I have done some research on it on various game boards, and most experienced players recommend not to take Storm's End, and offering to leave it to Martells on the condition that they don't muster units there. 

After a moment of thinking, I think it's about right. Storm's End is, after all, just one regular castle. But it basically confines Martells to Dorne, so it encourages betrayal, and since Dornish Sea is rather impregnable early game, it will suffer constant raiding orders from there, reducing its usefulness almost to zero. If Martells try to muster there, you can always raid them from Shipbreaker Bay anyway. Out of your two closest enemies - Martells and Starks, the former are usually not the weaker side, so striking an alliance with them seems like a better option.