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 $37.66   3.539%   $38.9928   $38.99   $39.00   $39.00  
  Purchase #2 $107.99   7.110%   $115.6681   $115.67   $115.65   $115.70  
  Purchase #3 $159.70   11.651%   $178.3066   $178.31   $178.30   $178.30  
  Purchase #4 $228.51   7.618%   $245.9179   $245.92   $245.90   $245.90  
  Purchase #5 $139.63   3.167%   $144.0521   $144.05   $144.05   $144.10  
  Purchase #6 $28.35   12.553%   $31.9088   $31.91   $31.90   $31.90  
  Purchase #7 $35.96   4.687%   $37.6454   $37.65   $37.65   $37.60  
  7 purchase totals = $737.80       $792.4917   $792.50   $792.45   $792.50