When you ride easy, and brake easy, the shifter cable will work as you know. My info is from Sheldon Brown:
Slight extension of the housing is not a problem with brakes. Sometimes the rear brake may drag slightly when the handlebars are turned all the way to one side, but you can't turn the bars that far when the bike is actually in motion. The small variation in housing length was too much for reliable indexed shifting, however, so
Shimano introduced "
S.I.S." housing, now widely copied by other manufacturers. This type of housing does not have a single helical-wound wire, but instead, it has a bundle of wires running pretty much straight along parallel to the housing. They are held in place by being sandwiched between the plastic housing liner and the plastic outer covering.
Parts of index-compatible cable | | | |
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plastic outer covering | longitudinal wires | plastic liner | inner cable |
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Index-compatible housing doesn't change length significantly as its curvature changes, and so the shifter is able to communicate the correct setting to the derailer, even as the handlebars are turned, and the loops of cable housing bounce up and down due to bumps.
Warning: Since index-compatible housing relies on plastic to hold it together, it is not as strong as conventional helical housing,
and should never be used for brakes! The loads applied to brake cables can easily cause index-compatible housing to rupture and burst, causing a complete and sudden loss of brake function.
Extra care must be used in routing index-compatible housing because it is also less flexible than conventional housing.
(You may be able to get away with helical housing with indexed shifters where there is little or no change in cable curvature. I use helical housing for the handlebar-end shifters on my Bike Friday folding bicycle. The cable's greater flexibility makes the bicycle easier to disassemble and pack for travel. My Bike Friday has an 8-sprocket cassette. A cassette with more sprockets would be more finicky, because the sprockets are closer together.
https://www.sheldonbrown.com/cables.html