In theory what is more efficient?

IF you go through terrain varying in altitude up and down, is it more efficient to put most effort in climbing than stop peddling on the downhill? Or just stroll(go up slowly) the hills and ride fast down hill?

Also if you arent riding what is the most comfortable foot position is it wher the pedals are at the same height as eachother?

For me often I will climb fast uphill and put no effort downhill because I know Ill be stopped by traffic or find it too dangerous to go that fast.