A lot of emotions have surfaced in this thread but it is up to the buyer to do his/her research before taking the leap. Machine control steering is not only a function of GPS quality but also the software/firmware algorithims, AND the other componets that make up the WHOLE system.  So debating only the GPS componet is not a complete debate.  Having said that I have not personally ran the 4 inch GPS systems rather just RTK and occasionally for grins and giggles have tried WAAS while spraying.  That activity keeps my emotions grounded as to why I have RTK especially since we have a few fields we need to gravity irrigate and thus have eliminated cultivator blight due to great repeatability and elimination of subjective human error both for us and any help.

Personally my brother and I have used the same AB line file and base station for 5 years and have never nudged or trimmed.  In fact the system has pointed out implement problems which has made us do a better job of setting up our 3 point mounted implements with a sysmetrical approach.  As long as calibration is on it has always been the implement and not the machine control if we are having a problem.  We have for the most part quit using a ruler to check things during the season unless we are specifically calibrating.  Every spring while digging in the ground checking stand counts we smile because we are finding remains of the previous years roots.  We are ridge tillers and use the rolling stump cutters (cotton cutters) to pluck most of the previous root crown out pre plant.

One other thing we have done to really help the 3 point mounted planter stay nuts on (to reduce possible implement drafting) are two large stabilizing coulters.  Our planter true V's don't have much depth or soil baring compared to other implements like cultivators, strip till and fertilizer rigs so the coulters took the wiggles out and made things look like a riffle shot.  Again this is splitting hairs but would still apply to RTK or the 4 inch systems.  Bottom line if we are on our game.....guess rows are very hard to find and we enjoy taking the subjective issues out of the equation by using RTK rather than something less.  My guess is if we where complete no till RTK would not be near as important and would decide that based on network availability vs satallite subscriptions.  We still however would treat equipment setup with the same vigilance and most likely if we would ever go awsay from mounted to pull type equipment we would put another complete system on that as well because of the same issues.

I hope that helps.