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 $12.39   5.664%   $13.0918   $13.09   $13.10   $13.10  
  Purchase #2 $89.81   11.344%   $99.998   $100.00   $100.00   $100.00  
  Purchase #3 $131.65   13.328%   $149.1963   $149.20   $149.20   $149.20  
  Purchase #4 $7.44   11.326%   $8.2827   $8.28   $8.30   $8.30  
  Purchase #5 $69.83   5.527%   $73.6895   $73.69   $73.70   $73.70  
  Purchase #6 $225.66   13.656%   $256.4761   $256.48   $256.50   $256.50  
  Purchase #7 $163.96   9.755%   $179.9543   $179.95   $179.95   $180.00  
  7 purchase totals = $700.74       $780.6887   $780.69   $780.75   $780.80