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 $217.37   10.464%   $240.1156   $240.12   $240.10   $240.10  
  Purchase #2 $98.68   6.425%   $105.0202   $105.02   $105.00   $105.00  
  Purchase #3 $14.49   7.475%   $15.5731   $15.57   $15.55   $15.60  
  Purchase #4 $7.45   4.330%   $7.7726   $7.77   $7.75   $7.80  
  Purchase #5 $91.47   10.636%   $101.1987   $101.20   $101.20   $101.20  
  Purchase #6 $153.90   3.686%   $159.5728   $159.57   $159.55   $159.60  
  Purchase #7 $10.53   4.113%   $10.9631   $10.96   $10.95   $11.00  
  7 purchase totals = $593.89       $640.2161   $640.21   $640.10   $640.30