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 $66.93   9.510%   $73.295   $73.30   $73.30   $73.30  
  Purchase #2 $5.91   13.499%   $6.7078   $6.71   $6.70   $6.70  
  Purchase #3 $184.99   11.233%   $205.7699   $205.77   $205.75   $205.80  
  Purchase #4 $117.92   12.516%   $132.6789   $132.68   $132.70   $132.70  
  Purchase #5 $83.73   3.708%   $86.8347   $86.83   $86.85   $86.80  
  Purchase #6 $242.44   6.254%   $257.6022   $257.60   $257.60   $257.60  
  Purchase #7 $31.25   12.707%   $35.2209   $35.22   $35.20   $35.20  
  7 purchase totals = $733.17       $798.1094   $798.11   $798.10   $798.10