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 $19.72   10.248%   $21.7409   $21.74   $21.75   $21.70  
  Purchase #2 $9.92   9.793%   $10.8915   $10.89   $10.90   $10.90  
  Purchase #3 $48.69   3.217%   $50.2564   $50.26   $50.25   $50.30  
  Purchase #4 $116.42   3.757%   $120.7939   $120.79   $120.80   $120.80  
  Purchase #5 $68.49   3.422%   $70.8337   $70.83   $70.85   $70.80  
  Purchase #6 $19.96   13.278%   $22.6103   $22.61   $22.60   $22.60  
  Purchase #7 $122.43   7.415%   $131.5082   $131.51   $131.50   $131.50  
  7 purchase totals = $405.63       $428.6349   $428.63   $428.65   $428.60