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 $81.37   8.583%   $88.354   $88.35   $88.35   $88.40  
  Purchase #2 $233.58   8.556%   $253.5651   $253.57   $253.55   $253.60  
  Purchase #3 $36.35   3.736%   $37.708   $37.71   $37.70   $37.70  
  Purchase #4 $52.91   6.267%   $56.2259   $56.23   $56.25   $56.20  
  Purchase #5 $229.66   4.703%   $240.4609   $240.46   $240.45   $240.50  
  Purchase #6 $109.99   8.177%   $118.9839   $118.98   $119.00   $119.00  
  Purchase #7 $14.47   7.787%   $15.5968   $15.60   $15.60   $15.60  
  7 purchase totals = $758.33       $810.8946   $810.90   $810.90   $811.00