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 $241.61   10.294%   $266.4813   $266.48   $266.50   $266.50  
  Purchase #2 $33.84   5.154%   $35.5841   $35.58   $35.60   $35.60  
  Purchase #3 $169.80   5.588%   $179.2884   $179.29   $179.30   $179.30  
  Purchase #4 $241.99   3.474%   $250.3967   $250.40   $250.40   $250.40  
  Purchase #5 $34.57   12.859%   $39.0154   $39.02   $39.00   $39.00  
  Purchase #6 $102.39   5.233%   $107.7481   $107.75   $107.75   $107.70  
  Purchase #7 $106.26   7.684%   $114.425   $114.43   $114.45   $114.40  
  7 purchase totals = $930.46       $992.9390   $992.95   $993.00   $992.90