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 $27.97   9.789%   $30.708   $30.71   $30.70   $30.70  
  Purchase #2 $17.42   4.296%   $18.1684   $18.17   $18.15   $18.20  
  Purchase #3 $81.46   10.441%   $89.9652   $89.97   $89.95   $90.00  
  Purchase #4 $131.89   12.860%   $148.8511   $148.85   $148.85   $148.90  
  Purchase #5 $192.28   7.429%   $206.5645   $206.56   $206.55   $206.60  
  Purchase #6 $124.50   10.195%   $137.1928   $137.19   $137.20   $137.20  
  Purchase #7 $10.48   3.506%   $10.8474   $10.85   $10.85   $10.80  
  7 purchase totals = $586.00       $642.2974   $642.30   $642.25   $642.40