There's been a discussion going on over at the PS Audio Community forums page about directionality of fuses.
Directionality is a controversial topic. For those unfamiliar with it, the basic idea is that cables and fuses sound different depending on the direction they are used. Reverse an RCA cable's direction or a speaker cable's direction and the system will sound different.
This is controversial because in most cases the cables or fuses are being used in AC paths which really don't care about directionality - yet we believe strongly in these audible differences.
I can tell you from personal experience that in some cases I can clearly hear a difference in cable directionality and one that's repeatable. In these case I mark the cable to reflect those changes.
In other instances I hear a difference but that difference is not repeatable and reliable. In those cases I chalk it up to contact cleaning. Contact cleaning happens every time you plug something in or out of your system - a small amount of oxidation is removed from the connector's contacts with every removal and insertion and you hear this change.
The challenge is to separate the two phenomenas: directional changes and contact cleaning.
The former is an important aspect of system setup, the latter a simple exercise to be noticed then ignored.