Here is the way I looked at it and I understand your logic and I did some branch runs as you mentioned. Basically in your home plumbing you will be dealing with .50 and .75 PEX I did the math and the IDs respectively are .475 and .677 that is roughly double the area or in terms of volume a 10 foot run of each respectively holds 21 and 43 cubic inches or 1.5 and 3.0 cups of water. So to purge a 30ft run you will waste 4.5 or 9.0 cups of water.
You also have to factor in the cost of fittings and clamp rings compared to the cost of tubing and then the cost of the shut off valves and connector lines as compared to running straight to the device and then a connection fitting.
I have my manifold centrally located in the basement so all first floor runs are in the 20 foot or less distance of the manifold. As an example I ran a hot and cold to the kitchen sink and branched off there for the dishwasher and fridge. I have an outside faucet within 5 feet of the sink supply that I could have also connected in but I chose to run back to the manifold for that. Reason being if someone is using the sink and someone turns on the hose the sink will go hot quicker than having the tempering distance of the run. At least that was my logic there. The run to the upstairs bath is the longest run and was harder to run maybe 30 feet and I had a .75 port on the end of the manifold I made so I chose the larger PEX size and ran a hot and cold with branches once to the bathroom. I did valves up stairs and a master valve in basement. Could I have lived with .50 running the whole bathroom most likely yes but in going with the .75 I knew I would have less loss when all the things were being used at once. True when Im in the shower and someone uses the sink they have less wait on hot water but then again every time I use the shower or sink alone Im purging twice the water than if I had run more smaller lines. If I had it to do again I might do the homerun to the upstairs bath with .50. When you buy your tubing it comes in rolls and I needed .75 to go from the meter to the water heater and then a hot and cold to supply the manifold. Those runs were not that long and I had enough .75 left from the roll to do the bath supply. Being cheap I didnt want to have 60 foot of tubing hanging in the garage forever so I used it up there.
I also like the peace of mind knowing there is no connections inside any closed in areas. Running this stuff is more like running wire than it is plumbing.
Here is a photo of the manifold I cobbled together some runs were not connected yet.
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