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 $241.39   13.245%   $273.3621   $273.36   $273.35   $273.40  
  Purchase #2 $65.52   12.500%   $73.71   $73.71   $73.70   $73.70  
  Purchase #3 $131.61   5.378%   $138.688   $138.69   $138.70   $138.70  
  Purchase #4 $61.47   5.616%   $64.9222   $64.92   $64.90   $64.90  
  Purchase #5 $215.27   8.128%   $232.7671   $232.77   $232.75   $232.80  
  Purchase #6 $227.38   12.235%   $255.1999   $255.20   $255.20   $255.20  
  Purchase #7 $199.81   10.410%   $220.6102   $220.61   $220.60   $220.60  
  7 purchase totals = $1,142.45       $1,259.2595   $1,259.26   $1,259.20   $1,259.30