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 $244.80   6.635%   $261.0425   $261.04   $261.05   $261.00  
  Purchase #2 $102.97   3.691%   $106.7706   $106.77   $106.75   $106.80  
  Purchase #3 $72.30   10.181%   $79.6609   $79.66   $79.65   $79.70  
  Purchase #4 $40.85   11.460%   $45.5314   $45.53   $45.55   $45.50  
  Purchase #5 $146.28   5.190%   $153.8719   $153.87   $153.85   $153.90  
  Purchase #6 $246.39   12.384%   $276.9029   $276.90   $276.90   $276.90  
  Purchase #7 $34.27   5.426%   $36.1295   $36.13   $36.15   $36.10  
  7 purchase totals = $887.86       $959.9097   $959.90   $959.90   $959.90