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 $9.98   4.361%   $10.4152   $10.42   $10.40   $10.40  
  Purchase #2 $25.63   8.418%   $27.7875   $27.79   $27.80   $27.80  
  Purchase #3 $203.87   12.185%   $228.7116   $228.71   $228.70   $228.70  
  Purchase #4 $76.93   12.329%   $86.4147   $86.41   $86.40   $86.40  
  Purchase #5 $75.48   3.736%   $78.2999   $78.30   $78.30   $78.30  
  Purchase #6 $24.47   10.792%   $27.1108   $27.11   $27.10   $27.10  
  Purchase #7 $74.95   4.476%   $78.3048   $78.30   $78.30   $78.30  
  7 purchase totals = $491.31       $537.0445   $537.04   $537.00   $537.00