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 $248.27   8.167%   $268.5462   $268.55   $268.55   $268.50  
  Purchase #2 $38.82   12.219%   $43.5634   $43.56   $43.55   $43.60  
  Purchase #3 $162.55   8.578%   $176.4935   $176.49   $176.50   $176.50  
  Purchase #4 $5.75   3.304%   $5.94   $5.94   $5.95   $5.90  
  Purchase #5 $57.93   10.364%   $63.9339   $63.93   $63.95   $63.90  
  Purchase #6 $157.88   7.721%   $170.0699   $170.07   $170.05   $170.10  
  Purchase #7 $249.35   12.105%   $279.5338   $279.53   $279.55   $279.50  
  7 purchase totals = $920.55       $1,008.0807   $1,008.07   $1,008.10   $1,008.00