Should be kept in the tile when writing in the blueprint and items should be added to the list of requirements, see report on http://mod-buildcraft.com/forums/index.php?topic=180.0;topicseen

Does this include chests? I wouldn't want to empty all chests before making a blueprint...

On the other hand, you may create systems that do requires certain items to be on chests. Trade-off and consistency is hard to find.