I would like to know this as well. Currently in my data center all kinds of shoes are worn. We have loafers, sneakers, combat boots, and sandals. We have an antistatic floor as well. I have not noticed any affect on our systems due to the different types of foot ware.
ideally speaking, the footwear does not have much of an impact in the data center. you only have to ensure that they are clean and do not bring in dust and dirt into the data center which when accumulated near the servers could cause unnecessary heating. I also would insist on rubber soles just to be on the safer side.