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 $21.37   6.660%   $22.7932   $22.79   $22.80   $22.80  
  Purchase #2 $42.35   10.428%   $46.7663   $46.77   $46.75   $46.80  
  Purchase #3 $233.70   13.275%   $264.7237   $264.72   $264.70   $264.70  
  Purchase #4 $167.30   3.775%   $173.6156   $173.62   $173.60   $173.60  
  Purchase #5 $114.57   9.366%   $125.3006   $125.30   $125.30   $125.30  
  Purchase #6 $238.34   7.652%   $256.5778   $256.58   $256.60   $256.60  
  Purchase #7 $183.54   8.714%   $199.5337   $199.53   $199.55   $199.50  
  7 purchase totals = $1,001.17       $1,089.3109   $1,089.31   $1,089.30   $1,089.30