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 $18.59   9.741%   $20.4009   $20.40   $20.40   $20.40  
  Purchase #2 $86.66   12.214%   $97.2447   $97.24   $97.25   $97.20  
  Purchase #3 $234.65   3.704%   $243.3414   $243.34   $243.35   $243.30  
  Purchase #4 $35.77   11.252%   $39.7948   $39.79   $39.80   $39.80  
  Purchase #5 $40.92   4.369%   $42.7078   $42.71   $42.70   $42.70  
  Purchase #6 $215.62   3.254%   $222.6363   $222.64   $222.65   $222.60  
  Purchase #7 $76.67   10.280%   $84.5517   $84.55   $84.55   $84.60  
  7 purchase totals = $708.88       $750.6776   $750.67   $750.70   $750.60