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 $74.87   5.207%   $78.7685   $78.77   $78.75   $78.80  
  Purchase #2 $24.53   7.421%   $26.3504   $26.35   $26.35   $26.40  
  Purchase #3 $56.99   11.465%   $63.5239   $63.52   $63.50   $63.50  
  Purchase #4 $112.88   4.685%   $118.1684   $118.17   $118.15   $118.20  
  Purchase #5 $35.50   6.284%   $37.7308   $37.73   $37.75   $37.70  
  Purchase #6 $44.75   10.657%   $49.519   $49.52   $49.50   $49.50  
  Purchase #7 $227.54   3.238%   $234.9077   $234.91   $234.90   $234.90  
  7 purchase totals = $577.06       $608.9687   $608.97   $608.90   $609.00