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 $120.83   12.729%   $136.2105   $136.21   $136.20   $136.20  
  Purchase #2 $180.55   12.414%   $202.9635   $202.96   $202.95   $203.00  
  Purchase #3 $61.49   5.154%   $64.6592   $64.66   $64.65   $64.70  
  Purchase #4 $244.46   3.774%   $253.6859   $253.69   $253.70   $253.70  
  Purchase #5 $106.46   3.794%   $110.4991   $110.50   $110.50   $110.50  
  Purchase #6 $59.68   12.391%   $67.0749   $67.07   $67.05   $67.10  
  Purchase #7 $82.94   10.751%   $91.8569   $91.86   $91.85   $91.90  
  7 purchase totals = $856.41       $926.9500   $926.95   $926.90   $927.10