I have seen many posts regarding rounding errors over the years but cannot find one describing this.

Bug example: Attribute price is 10 on Product Info page, but when added to cart it is calculated at 9.99.

Steps to duplicate this in a vanilla 1.54:

1) Currency Euro
2) Tax zone all countries, all zones
3) Tax rate 21%
4) Configuration->My Store: show prices with tax.
5) Edit the product attributes of:
Test 10% by Attrib
Hewlett Packard - by attributes SALE (pid 36).
Set white to 8.2644 (10 euros -21% tax).
6) View in shop: white attribute = 10 euros
7) Add to cart = 9.99.

I see the calculations for the attributes on the product info page and those in the cart are indeed handled by separate lumps of similar-looking but evidently not identical code in the attributes module and the shoppingcart class.
No doubt pegged for a reworking but any chance of a band-aid now?