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 $14.50   10.304%   $15.9941   $15.99   $16.00   $16.00  
  Purchase #2 $157.80   13.480%   $179.0714   $179.07   $179.05   $179.10  
  Purchase #3 $128.86   11.143%   $143.2189   $143.22   $143.20   $143.20  
  Purchase #4 $10.99   3.446%   $11.3687   $11.37   $11.35   $11.40  
  Purchase #5 $233.87   9.514%   $256.1204   $256.12   $256.10   $256.10  
  Purchase #6 $112.81   12.837%   $127.2914   $127.29   $127.30   $127.30  
  Purchase #7 $248.42   3.456%   $257.0054   $257.01   $257.00   $257.00  
  7 purchase totals = $907.25       $990.0703   $990.07   $990.00   $990.10