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 $84.91   7.704%   $91.4515   $91.45   $91.45   $91.50  
  Purchase #2 $111.37   11.522%   $124.2021   $124.20   $124.20   $124.20  
  Purchase #3 $25.73   5.591%   $27.1686   $27.17   $27.15   $27.20  
  Purchase #4 $132.31   3.383%   $136.786   $136.79   $136.80   $136.80  
  Purchase #5 $102.40   6.388%   $108.9413   $108.94   $108.95   $108.90  
  Purchase #6 $61.40   5.746%   $64.928   $64.93   $64.95   $64.90  
  Purchase #7 $26.85   6.108%   $28.49   $28.49   $28.50   $28.50  
  7 purchase totals = $544.97       $581.9675   $581.97   $582.00   $582.00