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 $139.26   13.472%   $158.0211   $158.02   $158.00   $158.00  
  Purchase #2 $200.62   3.116%   $206.8713   $206.87   $206.85   $206.90  
  Purchase #3 $128.80   5.731%   $136.1815   $136.18   $136.20   $136.20  
  Purchase #4 $54.86   6.615%   $58.489   $58.49   $58.50   $58.50  
  Purchase #5 $93.63   9.648%   $102.6634   $102.66   $102.65   $102.70  
  Purchase #6 $14.30   11.432%   $15.9348   $15.93   $15.95   $15.90  
  Purchase #7 $188.59   6.195%   $200.2732   $200.27   $200.25   $200.30  
  7 purchase totals = $820.06       $878.4343   $878.42   $878.40   $878.50