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 $15.50   10.596%   $17.1424   $17.14   $17.15   $17.10  
  Purchase #2 $227.41   9.812%   $249.7235   $249.72   $249.70   $249.70  
  Purchase #3 $8.87   13.781%   $10.0924   $10.09   $10.10   $10.10  
  Purchase #4 $7.96   12.664%   $8.9681   $8.97   $8.95   $9.00  
  Purchase #5 $181.34   10.728%   $200.7942   $200.79   $200.80   $200.80  
  Purchase #6 $115.75   11.836%   $129.4502   $129.45   $129.45   $129.50  
  Purchase #7 $29.90   4.167%   $31.1459   $31.15   $31.15   $31.10  
  7 purchase totals = $586.73       $647.3167   $647.31   $647.30   $647.30