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 $9.39   6.338%   $9.9851   $9.99   $10.00   $10.00  
  Purchase #2 $64.61   8.745%   $70.2601   $70.26   $70.25   $70.30  
  Purchase #3 $233.27   7.814%   $251.4977   $251.50   $251.50   $251.50  
  Purchase #4 $223.69   10.389%   $246.9292   $246.93   $246.95   $246.90  
  Purchase #5 $17.35   7.344%   $18.6242   $18.62   $18.60   $18.60  
  Purchase #6 $190.51   10.870%   $211.2184   $211.22   $211.20   $211.20  
  Purchase #7 $119.73   12.814%   $135.0722   $135.07   $135.05   $135.10  
  7 purchase totals = $858.55       $943.5869   $943.59   $943.55   $943.60