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 $28.46   6.180%   $30.2188   $30.22   $30.20   $30.20  
  Purchase #2 $6.34   7.268%   $6.8008   $6.80   $6.80   $6.80  
  Purchase #3 $148.30   10.122%   $163.3109   $163.31   $163.30   $163.30  
  Purchase #4 $117.93   12.542%   $132.7208   $132.72   $132.70   $132.70  
  Purchase #5 $184.88   6.128%   $196.2094   $196.21   $196.20   $196.20  
  Purchase #6 $128.58   8.674%   $139.733   $139.73   $139.75   $139.70  
  Purchase #7 $42.30   10.869%   $46.8976   $46.90   $46.90   $46.90  
  7 purchase totals = $656.79       $715.8913   $715.89   $715.85   $715.80