When you open up 10 different accordions you will likely find 10 different ways in which the valves have been done, different types, boosters or no boosters, leather or plastic, etc.
Companies have also changed their ways, based on experience. What I have learnt over the years is:
1) Avoid (faux) leather valves with strips of plastic as boosters. The plastic boosters may work well in the beginning (if they don't cause problems with resonating on large leathers for low notes right from the start) but over time they lose their strength to keep the valves closed, especially if they are hanging down for a long time (like in a cassotto instrument stored on its feet).
2) The best has always been and still is to use leather that is soft enough to open and close easily, with metal booster springs, supported at the base with a paper or leather circle that is large enough (I have 8mm leather, 5 and 6 mm paper). Use booster springs that are "just strong enough" to close the valves but not any stronger as that will impede the air flow.
3) Once you get to the higher notes where you start thinking that even the thinnest leather + the thinnest booster spring is offering too much resistance, switch to plastic valves. For me that is typically 3 or 4 notes before you stop using valves altogether. (In the end you thus only use 1 size of plastic valves: the smallest size.)
Some manufacturers have been "cutting corners" by not using booster springs inside the reed block for notes to which they do apply booster springs on the outside. That's not a good idea. Many manufacturers do not use booster springs on smaller valves that are still leather. Again not a good idea as the leather alone will lose its strength over time.
In any case the least you should do (and this is exactly what I found in an older very high quality Guerrini) is have metal boosters on *all* the valves that are hanging down when the accordion is stored on its feet. Leather valves without booster springs may do well enough outside cassotto (and not in Winkelbaß either) but definitely not in the cassotto.
A scheme with something like leather valve up to around C4 and plastic above is also fine (but the leather should be real leather with metal booster). Plastic valves work fine, but are a little bit noisier than leather valves.
The rule C#6 and above-> no valve is a very generally used rule (some start C6 and some only start from D6 to omit the valve). Very high reeds need the "spilled" air through the non-playing reed in order to start. It's not an exact science as to what note should be the cut-off and I have seen differences between valve or no valve on C6 and C#6 and even a bit lower then that, but that was then only on a Russian bayan.
