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 $183.99   13.612%   $209.0347   $209.03   $209.05   $209.00  
  Purchase #2 $239.73   13.838%   $272.9038   $272.90   $272.90   $272.90  
  Purchase #3 $73.65   13.812%   $83.8225   $83.82   $83.80   $83.80  
  Purchase #4 $172.39   12.356%   $193.6905   $193.69   $193.70   $193.70  
  Purchase #5 $121.74   9.304%   $133.0667   $133.07   $133.05   $133.10  
  Purchase #6 $51.94   8.338%   $56.2708   $56.27   $56.25   $56.30  
  Purchase #7 $33.69   4.724%   $35.2815   $35.28   $35.30   $35.30  
  7 purchase totals = $877.13       $984.0705   $984.06   $984.05   $984.10