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 $12.25   13.245%   $13.8725   $13.87   $13.85   $13.90  
  Purchase #2 $28.88   5.218%   $30.387   $30.39   $30.40   $30.40  
  Purchase #3 $163.94   8.816%   $178.393   $178.39   $178.40   $178.40  
  Purchase #4 $230.57   12.256%   $258.8287   $258.83   $258.85   $258.80  
  Purchase #5 $156.25   13.503%   $177.3484   $177.35   $177.35   $177.30  
  Purchase #6 $98.72   13.462%   $112.0097   $112.01   $112.00   $112.00  
  Purchase #7 $46.62   6.423%   $49.6144   $49.61   $49.60   $49.60  
  7 purchase totals = $737.23       $820.4537   $820.45   $820.45   $820.40