It's not really worth thinking about the pipe "sloshing" unless you are providing significantly less steam than required by your machines and can never fill your pipes. Sloshing only occurs when they are filling up or emptying, if you are creating and moving excess steam the pipes essentially always stay full and there are no issues moving steam where you want it on demand. Using the largest pipes possible means you have a larger buffer/tank in your network and can equalize them faster when something is drawing very heavily on it, it might take a little longer to initially fill up and stabilize (stop sloshing), but that's not actually a real issue on it's own. It's like using a chain of openblocks tanks as a pipe system, it doesn't work very well if you don't have a lot of fluid in there because the liquid spreads out and sloshes around, but if you are providing a steady amount of liquid equal to or greater than the amount being used, all the tanks stay full and you get fluid everywhere on the chain that needs it. The pipe valves aren't required at all unless you are trying to do something with like subnetworks of pipes and need directional flow control, or have a very long stretch of pipe to move smaller amounts of fluid from one location or another, they are not for stopping sloshing on a normal pipe -> machine network because it shouldn't be occuring.
This actually not at all unique to gregtech, Mekanism and Buildcraft liquid pipes and the first two stages of EnderIO Fluid Conduits also have volumes and some amount of sloshing or gravity effect on them. It's not something most people think about or notice because it's not really an issue in most cases.
The "glow sensors" you are asking about are just normal lights from project red/bluepower placed on a liquid detector cover which is a gregtech cover that emits a redstone signal when fluid is inside.
