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 $235.34   12.183%   $264.0115   $264.01   $264.00   $264.00  
  Purchase #2 $64.84   6.802%   $69.2504   $69.25   $69.25   $69.30  
  Purchase #3 $194.41   6.765%   $207.5618   $207.56   $207.55   $207.60  
  Purchase #4 $248.49   3.801%   $257.9351   $257.94   $257.95   $257.90  
  Purchase #5 $69.27   10.405%   $76.4775   $76.48   $76.50   $76.50  
  Purchase #6 $116.73   4.465%   $121.942   $121.94   $121.95   $121.90  
  Purchase #7 $88.38   4.337%   $92.213   $92.21   $92.20   $92.20  
  7 purchase totals = $1,017.46       $1,089.3913   $1,089.39   $1,089.40   $1,089.40