Well some people swear by them, but I am partial - many a customer I would hand off a clean build and give them a pack of the unused cables to store on their own initiative. Years later they contact me and they "lost" the unused cables and/or the hdd/dvd mounting brackets.
My bad, currently I zip-tie them to one of the unused 5.25 bays or secure them to the bottom of the case. Or if you do not have modular just route the unused cables out of the way and zip-tie them to the bundle toward the top of the case. Here is an old pic of my game rig...
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note all the power cables are in the background and only in the foreground as they migrate from the rear case fan onto the motherboard. The only interruptive airflow cables are the yellow IDE ribbons. The PSU is a PCP&C ATX510 non-modular and the unused ones are above the PSU itself.
Another example - my kid's PC
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Since there is no room above the PSU to organize I just zip-tied the wad of unused cables just outside the psu adjacent the DVD drive. The only interruptive airflow cables are the ribbon IDE and the PCIE power cables to the GPU. If the case had a top-mounted blowhole exhaust fan I probably would have chose one of the unused 5.25" bays to take up the excess cabling.
So in either case work needs to be done to get the cabling out of airflow as much as possible, regardless of a modular cable PSU.