It depends on the wooden frames your on about,some i reckon it does shorten the life of em or make them rot earlier than normal. If you've been doing it for 6 years they'll need a paint anyway I would think,a
Good test to see if it's effecting the houses you do is to look and see if it's peeling off all over the frames and not just on the odd part of the window,I know some of the houses I've done between me and the brush it has help them deteriorate quicker though.