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 $9.99   9.854%   $10.9744   $10.97   $10.95   $11.00  
  Purchase #2 $126.76   5.678%   $133.9574   $133.96   $133.95   $134.00  
  Purchase #3 $95.67   8.641%   $103.9368   $103.94   $103.95   $103.90  
  Purchase #4 $41.41   7.161%   $44.3754   $44.38   $44.40   $44.40  
  Purchase #5 $146.30   9.586%   $160.3243   $160.32   $160.30   $160.30  
  Purchase #6 $128.76   10.242%   $141.9476   $141.95   $141.95   $141.90  
  Purchase #7 $42.58   9.368%   $46.5689   $46.57   $46.55   $46.60  
  7 purchase totals = $591.47       $642.0848   $642.09   $642.05   $642.10