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 $124.60   9.297%   $136.1841   $136.18   $136.20   $136.20  
  Purchase #2 $182.53   10.201%   $201.1499   $201.15   $201.15   $201.10  
  Purchase #3 $244.48   13.387%   $277.2085   $277.21   $277.20   $277.20  
  Purchase #4 $239.87   11.285%   $266.9393   $266.94   $266.95   $266.90  
  Purchase #5 $180.29   6.268%   $191.5906   $191.59   $191.60   $191.60  
  Purchase #6 $149.50   11.477%   $166.6581   $166.66   $166.65   $166.70  
  Purchase #7 $79.63   7.849%   $85.8802   $85.88   $85.90   $85.90  
  7 purchase totals = $1,200.90       $1,325.6107   $1,325.61   $1,325.65   $1,325.60