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 $19.99   5.868%   $21.163   $21.16   $21.15   $21.20  
  Purchase #2 $50.86   4.164%   $52.9778   $52.98   $53.00   $53.00  
  Purchase #3 $5.71   13.466%   $6.4789   $6.48   $6.50   $6.50  
  Purchase #4 $155.51   6.174%   $165.1112   $165.11   $165.10   $165.10  
  Purchase #5 $198.63   11.503%   $221.4784   $221.48   $221.50   $221.50  
  Purchase #6 $190.26   12.252%   $213.5707   $213.57   $213.55   $213.60  
  Purchase #7 $190.89   3.188%   $196.9756   $196.98   $197.00   $197.00  
  7 purchase totals = $811.85       $877.7556   $877.76   $877.80   $877.90