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 $61.63   7.596%   $66.3114   $66.31   $66.30   $66.30  
  Purchase #2 $83.59   9.597%   $91.6121   $91.61   $91.60   $91.60  
  Purchase #3 $177.38   11.168%   $197.1898   $197.19   $197.20   $197.20  
  Purchase #4 $157.93   5.343%   $166.3682   $166.37   $166.35   $166.40  
  Purchase #5 $40.27   7.462%   $43.2749   $43.27   $43.25   $43.30  
  Purchase #6 $238.49   9.537%   $261.2348   $261.23   $261.25   $261.20  
  Purchase #7 $155.37   7.511%   $167.0398   $167.04   $167.05   $167.00  
  7 purchase totals = $914.66       $993.0310   $993.02   $993.00   $993.00