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 $159.92   3.308%   $165.2102   $165.21   $165.20   $165.20  
  Purchase #2 $120.96   6.738%   $129.1103   $129.11   $129.10   $129.10  
  Purchase #3 $87.46   3.129%   $90.1966   $90.20   $90.20   $90.20  
  Purchase #4 $209.34   13.291%   $237.1634   $237.16   $237.15   $237.20  
  Purchase #5 $115.69   6.520%   $123.233   $123.23   $123.25   $123.20  
  Purchase #6 $128.93   9.779%   $141.5381   $141.54   $141.55   $141.50  
  Purchase #7 $181.95   11.362%   $202.6232   $202.62   $202.60   $202.60  
  7 purchase totals = $1,004.25       $1,089.0748   $1,089.07   $1,089.05   $1,089.00