You have to decide on the sound that you want to achieve and whether or not you can execute it. Paranoid by Black Sabbath is a good example of a song that uses all down strokes in the verse chords and it doesn't quite sound right if you use alternate strokes. Highway Star by Deep Purple sounds like all down strokes in the "chugging" sections-I could never play it properly using all down strokes and thought I was "cheating" until I saw Blackmore playing it live and he was using alternating strokes. Iron Maiden uses different combinations depending on the effect they want and I am pretty sure that most of the signature "galloping" riffs use alternate strokes. As Firekorn has said, there is a limit on how fast you can pick using only down strokes. The more strings you try to play at once using alternate picking, the more difficult it becomes, so build up your alternate picking technique on chords one string at a time.