Personally, when I clean my lines, I flush with cleaner (homemade PBW) with super hot water, I actually boil it, and flush while it is uber hot, hot enough to pasteurize with cleaner (180°f). Then I rinse with cold RO water very well. If I need to store an untapped line, I will use RO or distilled water with Starsan mixed properly and then I charge the line accordingly. when I tap the line I run it until the beer flows. Usually there is some yeast initially, so I allow a few ounces of the beer/yeast to flow to ensure there is no Starsan left in the line.
This procedure has worked great.
FWIW, do not store cleaner in the lines, either, either sanitizer, or plain distilled water after a thorough cleansing.