Taps are normally specified at +- 2.5% and 5%. This would be 5.75 and 11.5 V in your case.
You can specify whatever taps you want - you would just need to agree this with the manufacturer. If may be worth talking to a manufacturer first if you want to do this.
You should look at what you want to achieve with the taps. We talking about nominal voltages anyhow, which will vary, so nothing will be that accurate anyhow.