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 $249.44   6.610%   $265.928   $265.93   $265.95   $265.90  
  Purchase #2 $106.54   4.690%   $111.5367   $111.54   $111.55   $111.50  
  Purchase #3 $114.38   4.183%   $119.1645   $119.16   $119.15   $119.20  
  Purchase #4 $95.48   3.303%   $98.6337   $98.63   $98.65   $98.60  
  Purchase #5 $211.42   8.263%   $228.8896   $228.89   $228.90   $228.90  
  Purchase #6 $141.85   12.196%   $159.15   $159.15   $159.15   $159.20  
  Purchase #7 $7.73   12.265%   $8.6781   $8.68   $8.70   $8.70  
  7 purchase totals = $926.84       $991.9806   $991.98   $992.05   $992.00