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 $34.88   10.213%   $38.4423   $38.44   $38.45   $38.40  
  Purchase #2 $229.56   8.655%   $249.4284   $249.43   $249.45   $249.40  
  Purchase #3 $198.36   4.296%   $206.8815   $206.88   $206.90   $206.90  
  Purchase #4 $154.92   8.771%   $168.508   $168.51   $168.50   $168.50  
  Purchase #5 $150.97   8.804%   $164.2614   $164.26   $164.25   $164.30  
  Purchase #6 $173.36   8.563%   $188.2048   $188.20   $188.20   $188.20  
  Purchase #7 $5.75   5.702%   $6.0779   $6.08   $6.10   $6.10  
  7 purchase totals = $947.80       $1,021.8043   $1,021.80   $1,021.85   $1,021.80