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 $11.89   12.522%   $13.3789   $13.38   $13.40   $13.40  
  Purchase #2 $131.25   13.589%   $149.0856   $149.09   $149.10   $149.10  
  Purchase #3 $76.87   5.191%   $80.8603   $80.86   $80.85   $80.90  
  Purchase #4 $142.53   4.432%   $148.8469   $148.85   $148.85   $148.80  
  Purchase #5 $154.94   5.744%   $163.8398   $163.84   $163.85   $163.80  
  Purchase #6 $23.60   4.623%   $24.691   $24.69   $24.70   $24.70  
  Purchase #7 $52.82   10.791%   $58.5198   $58.52   $58.50   $58.50  
  7 purchase totals = $593.90       $639.2223   $639.23   $639.25   $639.20