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 $22.91   13.154%   $25.9236   $25.92   $25.90   $25.90  
  Purchase #2 $124.96   10.442%   $138.0083   $138.01   $138.00   $138.00  
  Purchase #3 $204.26   11.392%   $227.5293   $227.53   $227.55   $227.50  
  Purchase #4 $125.63   9.718%   $137.8387   $137.84   $137.85   $137.80  
  Purchase #5 $232.79   12.514%   $261.9213   $261.92   $261.90   $261.90  
  Purchase #6 $78.56   5.730%   $83.0615   $83.06   $83.05   $83.10  
  Purchase #7 $37.56   13.833%   $42.7557   $42.76   $42.75   $42.80  
  7 purchase totals = $826.67       $917.0384   $917.04   $917.00   $917.00