Are Storefront API Costs reliable?

[vadimpk]

According to documentation found here

The costs are subject to change and changes will be reflected at checkout.

Can we actually rely on the costs calculated by the Cart API? I am asking because in some cases my app will not go through default Shopify checkout and since the Checkout API is deprecated, we need to use the Cart API.

I suspect that this line in documentation means a very basic thing, eg. that the cost can change when a customer selects a shipping rate at checkout – I'm fine with that. My concern is that Cart API can return a wrong amount, for example wrong tax or something like that, because that is a critical problem.
So, are the Cart API costs actually reliable?