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 $20.64   4.518%   $21.5725   $21.57   $21.55   $21.60  
  Purchase #2 $204.36   8.439%   $221.6059   $221.61   $221.60   $221.60  
  Purchase #3 $17.99   4.779%   $18.8497   $18.85   $18.85   $18.80  
  Purchase #4 $231.95   6.559%   $247.1636   $247.16   $247.15   $247.20  
  Purchase #5 $147.43   3.743%   $152.9483   $152.95   $152.95   $152.90  
  Purchase #6 $18.45   13.247%   $20.8941   $20.89   $20.90   $20.90  
  Purchase #7 $75.67   6.843%   $80.8481   $80.85   $80.85   $80.80  
  7 purchase totals = $716.49       $763.8822   $763.88   $763.85   $763.80