truly whichever is more convenient and gives you confidence, you should go by it. sometimes i cast directly upstream and take in the line, sometimes down and across, it depends on the situation. learn both ways
same here as clayton, usually up and across for dries and nymphs and down and across for streamers. but if the flow is not moving i will sometimes cast streamers directly across river and strip um in straight across too. have caught lots of bass that way cause the flows are always very very low in the summer.