You might want to try my Excel add-in, xlPrecision 2.0.
It avoids these errors by never converting to binary.
[....]
Greg Lovern
http://PrecisionCalc.com
Eliminate Hidden Spreadsheet Errors

Fascinating! I would imagine it slows computation
tremendously. Do you have any performance comparisons?

Would be better if Excel itself implemented BCD, at least
as an option. With nano-to-picosec instruction times, it is
probably a reasonable trade-off. I hope Bill is listening :-).