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 $74.71   12.167%   $83.8   $83.80   $83.80   $83.80  
  Purchase #2 $51.87   7.812%   $55.9221   $55.92   $55.90   $55.90  
  Purchase #3 $74.62   4.423%   $77.9204   $77.92   $77.90   $77.90  
  Purchase #4 $13.39   12.827%   $15.1075   $15.11   $15.10   $15.10  
  Purchase #5 $166.84   11.870%   $186.6439   $186.64   $186.65   $186.60  
  Purchase #6 $60.60   4.755%   $63.4815   $63.48   $63.50   $63.50  
  Purchase #7 $193.36   9.127%   $211.008   $211.01   $211.00   $211.00  
  7 purchase totals = $635.39       $693.8834   $693.88   $693.85   $693.80