residual valves?

[Panelfantastic]

Tried to search, no luck. Putting together 4whl disc for my panel.
What's the story with residual valves? How do you know when you need them? My understanding was, on lowered vehicles where the M/C could possibly be lower than the caliper, they are a necessity to prevent flow back into the M/C. but I've also read they cause "dragging" in some cases...
I'm also trying to learn about the porportioning valve but that seems a matter of personal choice/control than an absolute need (given the tire/wheel sizes are similar, from the posts I've read)
What are your experiences?