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 $245.47   10.281%   $270.7068   $270.71   $270.70   $270.70  
  Purchase #2 $169.82   13.514%   $192.7695   $192.77   $192.75   $192.80  
  Purchase #3 $92.29   8.424%   $100.0645   $100.06   $100.05   $100.10  
  Purchase #4 $60.88   10.216%   $67.0995   $67.10   $67.10   $67.10  
  Purchase #5 $236.63   9.613%   $259.3772   $259.38   $259.40   $259.40  
  Purchase #6 $185.45   6.531%   $197.5617   $197.56   $197.55   $197.60  
  Purchase #7 $173.28   8.809%   $188.5442   $188.54   $188.55   $188.50  
  7 purchase totals = $1,163.82       $1,276.1234   $1,276.12   $1,276.10   $1,276.20