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 $19.28   3.745%   $20.002   $20.00   $20.00   $20.00  
  Purchase #2 $134.53   10.610%   $148.8036   $148.80   $148.80   $148.80  
  Purchase #3 $16.53   9.232%   $18.056   $18.06   $18.05   $18.10  
  Purchase #4 $139.50   9.318%   $152.4986   $152.50   $152.50   $152.50  
  Purchase #5 $221.80   7.712%   $238.9052   $238.91   $238.90   $238.90  
  Purchase #6 $134.57   12.576%   $151.4935   $151.49   $151.50   $151.50  
  Purchase #7 $78.46   7.128%   $84.0526   $84.05   $84.05   $84.10  
  7 purchase totals = $744.67       $813.8115   $813.81   $813.80   $813.90