If the floor is properly ESD dissipative, you will need to ensure the following:
1) The mats need tested at installation.  If the mats are tied to a dissipative single point ground, they will not perform as you are hoping.
2) Assuming the mats pass installation testing, the mats will need to be bonded to the floor (E.g., carpet tape) to ensure that they are not sliding thus creating their own triboelectric effect or otherwise picking up debris which will create additional resistance as the ESD interface is compromised.
3) The mats will need to be added to your ESD verification protocol.

As a side note, and also an issue I have with the ANSI-ESD S20.20 standard.  Verification can be 1-and-done which I disagree with.  Unless we are talking permanent metal to permanent metal ground, anything that is dissipative may degrade over time and should be checked at a reasonable frequency.