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 $239.26   12.411%   $268.9546   $268.95   $268.95   $269.00  
  Purchase #2 $127.46   6.604%   $135.8775   $135.88   $135.90   $135.90  
  Purchase #3 $59.30   7.598%   $63.8056   $63.81   $63.80   $63.80  
  Purchase #4 $112.87   10.485%   $124.7044   $124.70   $124.70   $124.70  
  Purchase #5 $165.36   3.712%   $171.4982   $171.50   $171.50   $171.50  
  Purchase #6 $16.35   5.217%   $17.203   $17.20   $17.20   $17.20  
  Purchase #7 $19.66   9.430%   $21.5139   $21.51   $21.50   $21.50  
  7 purchase totals = $740.26       $803.5572   $803.55   $803.55   $803.60