> It's always integers unless you have a VERY good reason to do otherwise

I don't really agree. This seems like one of those outdated "greybeard rules" which people love to cargo cult, but to me it just comes with its own set of trade-offs, like now you have to think about exponents everywhere and getting them wrong comes with orders-of-magnitude consequences.

If you have a modern DB and your languages handle its decimals well then I'd use them. You can translate as needed for imports and exports, but the core of your system is always sound. Switch to the more basic formats if and only if there was some perf problem with decimals, which for most fintechs will be a VERY long time with modern DBs.

What really pushes me towards decimals is the consequence of getting something wrong. Relying on comparing raw ints, with variable exponents baked in, can lead to catastrophic errors with multi-order-of-magnitude gaps if that implicit exponent isn't carried along correctly. It's a massive footgun always waiting to happen. Decimals avoid that whole class of problem. So the goal is to maximise the "safe" decimals as source of truth everywhere you can, and have very well-tested "in and out" paths for any producers or consumers with different representation preferences. To me, that's good system design. And when, not if, you're manually inspecting DB records to track down a bug, having everything normalized in a glanceable, obvious format like decimals will let you recognise errors faster and more intuitively.

This goes extra for crypto, especially stablecoins, where one USD stablecoin might be exp-6 and another might be exp-9. And you're representing them in the same DB! Off by a factor of a thousand if you miss that exponent! Decimalize that immediately says I.