As part of an overall wiring system upgrade I'm planning, I'm considering replacing my stock fuse block with an ATO/ATC style block. The units I'm looking at have 12 to 18 circuits each, which will allow me to fuse most circuits individually (as well as to add circuits for things like a/c, a rear window defroster and an electric fan). I like this idea because it seems that it would make troubleshooting electrical faults considerably easier, by simplifying the overall wiring diagram. Are there downsides or pitfalls to consider in adopting this approach?

How does one decide on the correct amperage rating for individual circuits? I imagine there's a small but not insignificant risk of fire or of burning out an accessory with too-high fusing, while too low would tend to blow under normal loads. Is this correct? Eight amp ATO/ATC fuses aren't made and 25A fuses, while theoretically available, have proved so far impossible to find. So do I go one step lower (replace 8A fused circuits with 7.5A and 25A with 20A) or higher (replace 8A with 10A and 25A with 30A)?