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 $58.86   8.853%   $64.0709   $64.07   $64.05   $64.10  
  Purchase #2 $188.79   8.288%   $204.4369   $204.44   $204.45   $204.40  
  Purchase #3 $69.70   10.670%   $77.137   $77.14   $77.15   $77.10  
  Purchase #4 $64.50   11.531%   $71.9375   $71.94   $71.95   $71.90  
  Purchase #5 $245.57   9.373%   $268.5873   $268.59   $268.60   $268.60  
  Purchase #6 $240.78   13.643%   $273.6296   $273.63   $273.65   $273.60  
  Purchase #7 $24.55   12.712%   $27.6708   $27.67   $27.65   $27.70  
  7 purchase totals = $892.75       $987.4700   $987.48   $987.50   $987.40