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 $133.50   4.374%   $139.3393   $139.34   $139.35   $139.30  
  Purchase #2 $145.36   9.751%   $159.5341   $159.53   $159.55   $159.50  
  Purchase #3 $145.53   12.512%   $163.7387   $163.74   $163.75   $163.70  
  Purchase #4 $28.95   3.602%   $29.9928   $29.99   $30.00   $30.00  
  Purchase #5 $12.70   11.844%   $14.2042   $14.20   $14.20   $14.20  
  Purchase #6 $45.32   5.813%   $47.9545   $47.95   $47.95   $48.00  
  Purchase #7 $6.92   5.579%   $7.3061   $7.31   $7.30   $7.30  
  7 purchase totals = $518.28       $562.0697   $562.06   $562.10   $562.00