The expense tool at my current employer populates the rate based on the date of the transaction. For transaction amounts that I've encountered the default rate and the Visa rate are more or less similar. There is also the option to adjust the exchange rate manually on a per transaction basis.

I had a previous employer that also used a reference rate. If there were any deviations that showed a higher rate, one had to submit proof of the posted transaction amount. Again, without DCC, I didnt contest the rates because they evened out with Visa's rate.