can i make the base sauce with flour, butter, cheese and egg yolks ahead of time and reheat to mix with egg whites?
You can indeed make it ahead of time. I wouldn't use it refrigerator-cold, but it doesn't need to be hot when you fold in the whites. I often make it an hour or so in advance and leave at room temp until I'm ready to bake it. I don't add cheese to the base, however, but instead fold grated cheese into the base at the same time as the whipped whites (following Julia Child's instructions). If there's a lot of cheese in the base, you might need to heat it until hot to re-melt it.