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 $225.80   6.226%   $239.8583   $239.86   $239.85   $239.90  
  Purchase #2 $20.92   11.530%   $23.3321   $23.33   $23.35   $23.30  
  Purchase #3 $119.51   12.165%   $134.0484   $134.05   $134.05   $134.00  
  Purchase #4 $39.36   5.210%   $41.4107   $41.41   $41.40   $41.40  
  Purchase #5 $56.90   8.861%   $61.9419   $61.94   $61.95   $61.90  
  Purchase #6 $212.79   5.705%   $224.9297   $224.93   $224.95   $224.90  
  Purchase #7 $93.59   8.433%   $101.4824   $101.48   $101.50   $101.50  
  7 purchase totals = $768.87       $827.0035   $827.00   $827.05   $826.90