Cognac is definitely traditional (& in my opinion essential) for Steak Diane. However, you do not have to flambe it. Just add the cognac & simmer it down. Same difference without the drama or risk.
However, in all the times I've enjoyed this dish in restaurants or made it myself, I've never come across or seen a recipe that asked for tomato sauce. That simply isn't Steak Diane, nor is it traditional to the sauce. Traditional basic ingredients are butter, cream, shallots, sliced mushrooms, freshly ground black pepper, & cognac or brandy.
Goodweed - exactly how many recipes did you come across that claim tomato sauce is an ingredient of a "Diane" sauce? I've really never seen that before, in recipes or in restaurants.