The idea of clamping the brake hose and opening the bleed nipple is so that the old fluid displaced by retracting the piston is expelled out of the system, this way the argument is that there's no risk of any contaminants trapped in the fluid returning to e.g the ABS modulator when the piston is retracted. Having said that, I've never observed this practice myself when retracting caliper pistons and unless somebody can tell me otherwise I can't really see why the fluid would be degraded more at the caliper than the modulator/ master cylinder, although it has been said (I think) that the rubber flexi hoses at a microscopic level do gradually allow moisture to pass through and degrade the fluid, plus the caliper is at the 'dirty' end of the system that gets hot and dusty, plus the calipers are at the low point of the system where any fluid-borne dirt and debris could settle. Up to you really, if you bother having the brake fluid changed fairly regularly (as I do) then I can't really see it being an issue.
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