You should leave them in reverse mode until you are absolutely certain stopping is assured in case of brake failure, so there is no harm in leaving them in reverse idle until 20kts just in case you need them. I find I have to convince several new pilots you should do this as they get very nervous about any reverse below 60kts- they all want to frantically cancel it too early.
B737 FCTM - "The thrust levers should be positioned to reverse idle by taxi speed, then to full down after the engines have decelerated to idle."
If the brakes have failed you would know well before 20 kts.
Unless every pilot is instructed to do this you will spend you hole life teaching.