I've never had any problem at all with the red window on a Nettar, and I shoot in some very bright conditions routinely. Fast film, slow film, color, b&w, various brands, doesn't seem to matter. I wouldn't try it with 220 though. :-)
Of course with 6x6, there's the advantage that the red window is in the middle rather than at an edge, so it would take a pretty strange path to result in a light leak around the backing paper. I suppose in theory you could leave the window open for a long time and get print-through, but it's never been a problem for me.
Actually, apart from damaged bellows (bellowses? bellowsen?), the only leaks I've ever had in folders have been from the body "seam" rather than the back. Some of the older 6x9 models from other manufacturers don't close very solidly, but Zeiss certainly never had a problem with that!