Keeping your fly lines clean and lubricated is imperative to their performance.
I've been caught out on the water with a dried-out or dirty line and it can be a real hassle. Not only can excess friction from an improperly-maintained line cause casting headaches, but dirt/grime can even cause a floating line to not float as well. It's wise to pay attention to how your line is performing each trip so you know when a cleaning may be in order before things get worse, but it's also good to just do it periodically to keep the line in optimum shape. How often this needs to happen varies depending on how much and where you're fishing, but if you're fishing in scummy water or walking the banks a lot like I do, the line will be exposed to more junk and likely need cleaning more often. Thankfully, sinking lines (or the sinking portions of lines) don't require as much TLC and to my knowledge line dressings are typically not used on them as the sinking characteristics can be hampered.
A couple of weeks ago, I stumbled upon a great two-part video series from our friends at RIO Products that showcases a very detailed method of thoroughly cleaning a fly line. Whether you choose to go about it the same way or have a slightly different method of cleaning and dressing the line, I think you'll find the content interesting to watch. These clips are not exactly new, but they lay it all out nicely...check it!