Totally depends on the cell size and capacity you need. I mandate for our surveys that 12.5mW tx power and that 12 mb/s be the maditory rate while also allowing clients to drop down to 9mb/s needed. This keeps my cells small. This is good in our environments because we often have a high density of clients. Also you can adjust the receive Sensitivity threshold.
You should do some testing. Many types of clients do not like it when you lower the tx rate down that much.