Hm, I think that totally depends on where you are and your weather conditions. I for example have literally all of mine in full sun south windows, including paphs and phals. So far, no sunburn on anything. The only shade is from other plants. But I live way up north where the winters are very dark and even when the sun does glare down, like in the past couple of weeks, the angle greatly reduces the intensity that gets down here.
Just going as far south as New York City in the USA would make a huge difference (for comparison I am at approximately the latitude of Edmonton, Alberta in Canada).
So combine the latitude with the jet stream (i.e. lots of English rain), I let my orchids get at much sun as they can take, hence the south window 75% of the year.