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 $25.29   6.293%   $26.8815   $26.88   $26.90   $26.90  
  Purchase #2 $73.81   10.404%   $81.4892   $81.49   $81.50   $81.50  
  Purchase #3 $38.81   10.216%   $42.7748   $42.77   $42.75   $42.80  
  Purchase #4 $156.33   3.813%   $162.2909   $162.29   $162.30   $162.30  
  Purchase #5 $101.45   6.559%   $108.1041   $108.10   $108.10   $108.10  
  Purchase #6 $153.64   3.291%   $158.6963   $158.70   $158.70   $158.70  
  Purchase #7 $59.46   3.773%   $61.7034   $61.70   $61.70   $61.70  
  7 purchase totals = $608.79       $641.9402   $641.93   $641.95   $642.00