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 $29.92   8.682%   $32.5177   $32.52   $32.50   $32.50  
  Purchase #2 $216.38   13.611%   $245.8315   $245.83   $245.85   $245.80  
  Purchase #3 $18.83   10.831%   $20.8695   $20.87   $20.85   $20.90  
  Purchase #4 $108.36   6.128%   $115.0003   $115.00   $115.00   $115.00  
  Purchase #5 $205.97   3.186%   $212.5322   $212.53   $212.55   $212.50  
  Purchase #6 $163.69   13.311%   $185.4788   $185.48   $185.50   $185.50  
  Purchase #7 $28.68   5.702%   $30.3153   $30.32   $30.30   $30.30  
  7 purchase totals = $771.83       $842.5453   $842.55   $842.55   $842.50