It's always difficult to make statements that apply to both European and North American light. It has always been my practice to use a filter (for years it was an orange one, the last few years I have switched to yellow/green) so I have no examples myself of unfiltered landscape shots, but I was told by Fay Godwin, for example, that she seldom if ever uses filters. Looking back at a collection of her work, many pictures have been taken in overcast weather, or early in the morning/late in the afternoon or evening and appear not to have been filtered - they have sky tone present but it is very pale. Others have apparently been taken closer to midday and look as if they HAVE been filtered! If, in the conditions you commonly photograph in, you need a filter to get the sky tone you like, it's obviously good to use one, but I don't feel filters are as vital as they were!