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 $13.30   4.440%   $13.8905   $13.89   $13.90   $13.90  
  Purchase #2 $21.49   10.857%   $23.8232   $23.82   $23.80   $23.80  
  Purchase #3 $53.30   4.325%   $55.6052   $55.61   $55.60   $55.60  
  Purchase #4 $35.36   10.736%   $39.1562   $39.16   $39.15   $39.20  
  Purchase #5 $234.33   12.119%   $262.7285   $262.73   $262.75   $262.70  
  Purchase #6 $137.74   9.529%   $150.8652   $150.87   $150.85   $150.90  
  Purchase #7 $106.78   5.489%   $112.6412   $112.64   $112.65   $112.60  
  7 purchase totals = $602.30       $658.7100   $658.72   $658.70   $658.70