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 $249.46   5.731%   $263.7566   $263.76   $263.75   $263.80  
  Purchase #2 $19.69   9.140%   $21.4897   $21.49   $21.50   $21.50  
  Purchase #3 $244.31   5.572%   $257.923   $257.92   $257.90   $257.90  
  Purchase #4 $20.87   8.323%   $22.607   $22.61   $22.60   $22.60  
  Purchase #5 $50.84   10.863%   $56.3627   $56.36   $56.35   $56.40  
  Purchase #6 $10.56   8.299%   $11.4364   $11.44   $11.45   $11.40  
  Purchase #7 $21.81   9.242%   $23.8257   $23.83   $23.85   $23.80  
  7 purchase totals = $617.54       $657.4011   $657.41   $657.40   $657.40