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 $20.41   3.218%   $21.0668   $21.07   $21.05   $21.10  
  Purchase #2 $81.30   9.316%   $88.8739   $88.87   $88.85   $88.90  
  Purchase #3 $50.46   9.841%   $55.4258   $55.43   $55.45   $55.40  
  Purchase #4 $55.42   4.815%   $58.0885   $58.09   $58.10   $58.10  
  Purchase #5 $115.79   3.264%   $119.5694   $119.57   $119.55   $119.60  
  Purchase #6 $61.73   5.602%   $65.1881   $65.19   $65.20   $65.20  
  Purchase #7 $118.63   12.361%   $133.2939   $133.29   $133.30   $133.30  
  7 purchase totals = $503.74       $541.5064   $541.51   $541.50   $541.60