As a general rule, enthusiasts fiddling around with their HiFi is a harmless exercise, and if a few buy nonsense products that make them feel better, but have no effect on the sound, then there isn't much harm in that.
Fuses are different. They perform a crucial safety function, and all fuses are not equal. Putting to one side whether 'HiFi' fuses meet any sort of spec, even fuses of apparently the same rating from reputable manufacturers perform very differently according to their specific design. Slow blow, quick blow, motor start, cable protection - there is much more going on in a fuse than just a bit of wire in a glass tube. Googling will bring up mountains of information on fuse specification.
I would strongly recommend against replacing either the fuse in the plug, or an internal fuse in an amp (or any mains powered equipment come to that) with a fuse that isn't exactly as the equipment manufacturer specified.