power and fusing for first mains-powered project (100x WS2811 (RGB LED))
A fuse is first and foremost to protect the wiring. It then offers you some protection against dropped bits of wire if you are 'adjusting live'. But where semi-conductors are involved, it's unlikely to protect the equipment itself.

Your wiring from wall to PSU must be fuse protected, consistent with the size of wire you are using. 'Soldering for electric guitars', while a good skill, is not really sufficient qualification for dealing with mains-side wiring. You need to add extra care, and complying with the mains regulations for your region. If your wiring from wall to PSU is large enough to be protected by the fuse that you will have back in your distribution fuse box, say a 10A distribution fuse and 1mm2 wiring, then you do not need a mains fuse. If you want an additional lower value mains-side fuse, then buy a fusable plug, or shrouded fuse holder, or mount the fuse securely (not hot-melt glue) inside your plastic box. A 6A fuse will be adequate for down to 0.5mm2 conductor. A smaller fuse, down to 1A even, will be better at offering some slight protection to the PSU as well.

That particular power supply is protected against overcurrent on the output, as will the vast majority of SMPSs be. This means it's unlikely to blow a mains-input fuse without itself being in trouble first. It also means you don't need an output fuse if your DC wiring is at least 2mm2. A 6A fuse, for a 5.5A load is a bit 'close', and might run hot, drop excessive voltage, and nuisance blow. If you do want to fuse your DC side, then 10A, or a 6A fuse on each chain, might be better.