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 $30.25   4.829%   $31.7108   $31.71   $31.70   $31.70  
  Purchase #2 $129.97   7.462%   $139.6684   $139.67   $139.65   $139.70  
  Purchase #3 $95.56   3.475%   $98.8807   $98.88   $98.90   $98.90  
  Purchase #4 $87.90   12.501%   $98.8884   $98.89   $98.90   $98.90  
  Purchase #5 $17.70   12.193%   $19.8582   $19.86   $19.85   $19.90  
  Purchase #6 $102.81   5.329%   $108.2887   $108.29   $108.30   $108.30  
  Purchase #7 $171.25   8.629%   $186.0272   $186.03   $186.05   $186.00  
  7 purchase totals = $635.44       $683.3224   $683.33   $683.35   $683.40