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 $15.42   5.286%   $16.2351   $16.24   $16.25   $16.20  
  Purchase #2 $203.42   11.792%   $227.4073   $227.41   $227.40   $227.40  
  Purchase #3 $11.59   13.140%   $13.1129   $13.11   $13.10   $13.10  
  Purchase #4 $217.98   8.191%   $235.8347   $235.83   $235.85   $235.80  
  Purchase #5 $107.72   11.429%   $120.0313   $120.03   $120.05   $120.00  
  Purchase #6 $21.36   10.164%   $23.531   $23.53   $23.55   $23.50  
  Purchase #7 $10.46   12.814%   $11.8003   $11.80   $11.80   $11.80  
  7 purchase totals = $587.95       $647.9526   $647.95   $648.00   $647.80