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 $122.53   13.631%   $139.2321   $139.23   $139.25   $139.20  
  Purchase #2 $226.66   11.707%   $253.1951   $253.20   $253.20   $253.20  
  Purchase #3 $52.46   7.169%   $56.2209   $56.22   $56.20   $56.20  
  Purchase #4 $118.45   5.319%   $124.7504   $124.75   $124.75   $124.80  
  Purchase #5 $9.91   7.274%   $10.6309   $10.63   $10.65   $10.60  
  Purchase #6 $10.39   6.110%   $11.0248   $11.02   $11.00   $11.00  
  Purchase #7 $226.32   5.228%   $238.152   $238.15   $238.15   $238.20  
  7 purchase totals = $766.72       $833.2062   $833.20   $833.20   $833.20