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 $30.98   11.619%   $34.5796   $34.58   $34.60   $34.60  
  Purchase #2 $94.51   5.344%   $99.5606   $99.56   $99.55   $99.60  
  Purchase #3 $103.96   3.295%   $107.3855   $107.39   $107.40   $107.40  
  Purchase #4 $26.99   12.863%   $30.4617   $30.46   $30.45   $30.50  
  Purchase #5 $248.81   13.291%   $281.8793   $281.88   $281.90   $281.90  
  Purchase #6 $165.49   4.771%   $173.3855   $173.39   $173.40   $173.40  
  Purchase #7 $172.25   6.408%   $183.2878   $183.29   $183.30   $183.30  
  7 purchase totals = $842.99       $910.5400   $910.55   $910.60   $910.70