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 $17.42   10.820%   $19.3048   $19.30   $19.30   $19.30  
  Purchase #2 $190.62   13.871%   $217.0609   $217.06   $217.05   $217.10  
  Purchase #3 $69.71   11.341%   $77.6158   $77.62   $77.60   $77.60  
  Purchase #4 $76.42   7.150%   $81.884   $81.88   $81.90   $81.90  
  Purchase #5 $235.86   12.524%   $265.3991   $265.40   $265.40   $265.40  
  Purchase #6 $56.44   13.781%   $64.218   $64.22   $64.20   $64.20  
  Purchase #7 $107.72   9.196%   $117.6259   $117.63   $117.65   $117.60  
  7 purchase totals = $754.19       $843.1085   $843.11   $843.10   $843.10