Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $10.34   11.789%   $11.559   $11.56   $11.55   $11.60  
  Purchase #2 $210.82   4.459%   $220.2205   $220.22   $220.20   $220.20  
  Purchase #3 $108.41   12.752%   $122.2344   $122.23   $122.25   $122.20  
  Purchase #4 $98.94   11.499%   $110.3171   $110.32   $110.30   $110.30  
  Purchase #5 $76.52   10.505%   $84.5584   $84.56   $84.55   $84.60  
  Purchase #6 $18.92   8.699%   $20.5659   $20.57   $20.55   $20.60  
  Purchase #7 $60.47   12.850%   $68.2404   $68.24   $68.25   $68.20  
  7 purchase totals = $584.42       $637.6957   $637.70   $637.65   $637.70