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 $172.95   8.694%   $187.9863   $187.99   $188.00   $188.00  
  Purchase #2 $18.29   9.246%   $19.9811   $19.98   $20.00   $20.00  
  Purchase #3 $221.43   12.521%   $249.1553   $249.16   $249.15   $249.20  
  Purchase #4 $155.63   7.698%   $167.6104   $167.61   $167.60   $167.60  
  Purchase #5 $17.27   9.569%   $18.9226   $18.92   $18.90   $18.90  
  Purchase #6 $133.63   4.583%   $139.7543   $139.75   $139.75   $139.80  
  Purchase #7 $115.43   4.579%   $120.7155   $120.72   $120.70   $120.70  
  7 purchase totals = $834.63       $904.1255   $904.13   $904.10   $904.20