I think that might depend somewhat on what the stream is. For instance I watch quite a bit of Zelda Ocarina of Time Randomizer race restreams on Twitch. For those it is much much better to have some up-front ads when I join the stream, which is almost always during the "stream starts in 15 minutes" countdown, rather than to have ads appear mid-race. I don't know how much control the streamer has over this kind of thing, though.