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 $18.73   11.249%   $20.8369   $20.84   $20.85   $20.80  
  Purchase #2 $193.80   10.110%   $213.3932   $213.39   $213.40   $213.40  
  Purchase #3 $29.54   3.742%   $30.6454   $30.65   $30.65   $30.60  
  Purchase #4 $50.76   7.773%   $54.7056   $54.71   $54.70   $54.70  
  Purchase #5 $15.46   4.113%   $16.0959   $16.10   $16.10   $16.10  
  Purchase #6 $115.44   4.190%   $120.2769   $120.28   $120.30   $120.30  
  Purchase #7 $208.84   7.632%   $224.7787   $224.78   $224.80   $224.80  
  7 purchase totals = $632.57       $680.7326   $680.75   $680.80   $680.70