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 $71.66   13.296%   $81.1879   $81.19   $81.20   $81.20  
  Purchase #2 $183.82   10.365%   $202.8729   $202.87   $202.85   $202.90  
  Purchase #3 $234.57   7.854%   $252.9931   $252.99   $253.00   $253.00  
  Purchase #4 $123.29   10.499%   $136.2342   $136.23   $136.25   $136.20  
  Purchase #5 $66.27   12.123%   $74.3039   $74.30   $74.30   $74.30  
  Purchase #6 $105.64   8.835%   $114.9733   $114.97   $114.95   $115.00  
  Purchase #7 $153.54   3.265%   $158.5531   $158.55   $158.55   $158.60  
  7 purchase totals = $938.79       $1,021.1184   $1,021.10   $1,021.10   $1,021.20