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 $20.91   4.259%   $21.8006   $21.80   $21.80   $21.80  
  Purchase #2 $76.57   5.819%   $81.0256   $81.03   $81.05   $81.00  
  Purchase #3 $60.98   10.803%   $67.5677   $67.57   $67.55   $67.60  
  Purchase #4 $21.81   10.215%   $24.0379   $24.04   $24.05   $24.00  
  Purchase #5 $35.71   6.745%   $38.1186   $38.12   $38.10   $38.10  
  Purchase #6 $107.38   9.613%   $117.7024   $117.70   $117.70   $117.70  
  Purchase #7 $178.67   8.282%   $193.4674   $193.47   $193.45   $193.50  
  7 purchase totals = $502.03       $543.7202   $543.73   $543.70   $543.70