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 $130.80   5.791%   $138.3746   $138.37   $138.35   $138.40  
  Purchase #2 $18.80   9.186%   $20.527   $20.53   $20.55   $20.50  
  Purchase #3 $169.54   7.636%   $182.4861   $182.49   $182.50   $182.50  
  Purchase #4 $34.41   12.488%   $38.7071   $38.71   $38.70   $38.70  
  Purchase #5 $220.50   10.865%   $244.4573   $244.46   $244.45   $244.50  
  Purchase #6 $205.63   11.625%   $229.5345   $229.53   $229.55   $229.50  
  Purchase #7 $30.35   13.678%   $34.5013   $34.50   $34.50   $34.50  
  7 purchase totals = $810.03       $888.5879   $888.59   $888.60   $888.60