Is this done often, when putting a bike together? The reason I ask is that if you know what range of speed you will do most of your riding, you could select cogs to minimize the amount of shifting from small to big chainring.
I do notice that most 10-speed cassettes come set up where the six smallest cogs are only one tooth apart, I'm guessing because bike makers guess that's where you're doing the most riding, and it makes sense to have the gears close together in the riding range.
Where cassettes deviate the most are with the number of teeth on the four biggest cogs. One question I'm wondering, is there a rule on how many teeth apart that the four biggest cogs should not exceed (example, going from 21 cog to 24 cog is three apart, 24 to 28 cog is four apart)?
The main question is whether any of you had custom cassettes built and what were you trying to achieve with the custom cassette? It would be interesting to hear the cog-selection you chose, and how it turned out.