Hey Hank. Basically along the lines of what they all said, but will add the detail that it's usually not necessary to change them for strictly street running in most cases.
I ran mine with 3.5" of lift for quite a long time back when it was my daily-driver. But on trails, as they said, the cables were most definitely limiting wheel travel. Luckily I never damaged any of the brackets or anything like that, but I wasn't getting my full droop either.
Now, if you're prone to finding places around town to get some air
, then longer ones might be called for even for the street!
Or if your old ones are worn out or tired and need to be changed anyway, there's no reason to
not go longer at the same time.
Paul