At very low charge/discharge C rates, the risks are proportionally smaller. But a mismatch in capacity or resistance between parallel cells will cause the cell with the higher capacity or lower resistance to carry a disproportional amount of the load and charge current, causing accelerated degradation. If that degradation causes lithium plating and dendrite development then the degraded cell could eventually short. Without a fuse, that could be catastrophic. The cell will also run hotter as it degrades quicker, causing it to deliver higher capacity than the cooler cell(s), increasing the imbalance over time. I would fuse each individual cell. And take into account in my design that the fuses themselves introduce series resistance.
There is nothing more terrifying than a fire at sea.
For parallel batteries, the concern is eddie currents between the batteries. In some scenarios, the eddie currents can be enormous, unforeseen and undetectable. I have several batteries in parallel in my boat, but each is fused at its terminals with 0.5C rated fuses, and my maximum charge/discharge currents are below the rating of one single battery (0.1C for the whole battery array). Here's a discussion about parallel connected battery limitations:
https://www.master-instruments.com.au/tech-talk-article/3/Explaining the limits of LiFePO4 batteries in parallel.html
