I don't clean as often as I should, from memory gerber recommend every 5 metres or so and also everytime you change a foil. As Fred says use 99%+ isopropyl as any less will have water in it, what happens is the water drys up and the dirt left behind blocks the hole and you end up with a pixel wide line all along your work, and your left doing nothing but small stickers and trying to miss the line.
regards
Tel