You won't have to do anything different because of your 5ppm nitrate(NO3) in your tap water. Once you are cycled, you will begin your weekly habit of a gravel-clean-water-change and other maintenance. A weekly nitrate(NO3) test for the first few months can teach you whether this maintenance habit is being effective enough. The most important thing is that nitrate(NO3) readings settle down and stay steady, rather than keep creeping up.
A typical example might be that someone with zero ppm NO3 in there tap might want tank NO3 to settle in somewhere at or below 15 or 20ppm NO3. In your tank you'd want it to settle in at 20 to 25ppm since you'd be starting with an extra 5ppm. Actally, even if it settled in at 40ppm you'd probably not do anything different.
I agree with Chris that a good starting point for your thinking about water changing is the idea of a 50% change cutting an ammonia value in half as an example. But its also good to realize that the effect of a water change is sometimes non-linear in the sense that a "dirtier" tank can be more stubborn about retaining substances in the filter and substrate even "through" the gravel-clean-water-change process and therefor may not test out in quite so predictable a fashion, so testing is still quite an important part of the process. As he said, you will figure it our for your tank simply by water changing, testing, logging the results so you can look back at them and then just paying attention!
As the others have said, at this stage you are in a full-blown fish-in situation with probably a lot of testing, water-changing and figuring out to do to keep your tank working its ammonia and nitrite numbers between zero and 0.25ppm and that will probably keep you frustratingly busy for a while. It will settle down eventually though!
~~waterdrop~~