I use picnic taps and 10' lines which are always on the kegs in the fridge. When pouring, there is a noticeable transition in carbonation when the beer from the lines switches to new beer pulled from the keg. But even with 10' lines, this is about half a small serving. So I never bother. The new beer which come in still supplies a sufficient head. No ill effects seen on taste and mouthfeel. I doubt the beer in the line is completely flat, but definitely lower than normal. I never pitch it.
My father says, "hobbies are supposed to cost money".