What I usually tell folks depends on their past experiences. For example, if you're familiar with many different types of fly lines from different manufacturers, than choose a fly line that you feel will best enable the rod to do what you want it to do (i.e. dry fly fishing, bass fishing, etc).
However, if you're not up on all the latest fly lines, like most of us, than I always recommend that you use the same make of fly line you've been using on your other rods - in the appropriate weight for the new rod, of course. That way you'll be able to really compare your new rod to your other ones & have a good idea of how your rod will perform. If you've always been a fan of, say, Scientific Anglers for example, than use their fly line on your new rod - to get started at least. Later on you can start to experiment with different types of fly lines.
More than one of my bamboo & fiberglass clients have used the Cortland "Sylk" fly line - a synthetic line made to mimic the old silk lines, but without all the care & maintenance required. My past experiences find nothing wrong with these lines & I think they perform very well, but then again many lines will perform well.
Finally, if you're not very sure about which fly line to put on any rod, I suggest you take your fly rod with you on a visit to a local fly shop with a good reputation - one that you trust. The folks at almost any decent fly shop will gladly take the time to walk you through all the different types of lines you could use, allow you to compare them (even cast them), & offer you expert advice to help you decide. You can almost never go wrong at a small, local, independent fly shop.