@amchandler it depends on your age and whether you are contemplating distributions.
if you are over 59.5 years old (and the Roth has been opened for at least 5 calendar years) , it does not matter - any and all distributions are tax free.
if under 59.5 years old, then is MAY matter. Contribution dollars and Conversion dollars are tax free and penalty free (because the account has been open for at least 5 calendar years). These monies are assumed to be distributed before earnings. If you distribute earnings, they are subject to tax and the 10% penalty. That would be the reason to keep track of contributions (and conversions) - to determine that you have NOT distributed earnings, so it's a tax free distribution.