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 $208.51   12.463%   $234.4966   $234.50   $234.50   $234.50  
  Purchase #2 $27.64   3.732%   $28.6715   $28.67   $28.65   $28.70  
  Purchase #3 $163.76   5.510%   $172.7832   $172.78   $172.80   $172.80  
  Purchase #4 $198.41   4.407%   $207.1539   $207.15   $207.15   $207.20  
  Purchase #5 $248.50   12.773%   $280.2409   $280.24   $280.25   $280.20  
  Purchase #6 $242.77   7.608%   $261.2399   $261.24   $261.25   $261.20  
  Purchase #7 $45.39   9.614%   $49.7538   $49.75   $49.75   $49.80  
  7 purchase totals = $1,134.98       $1,234.3398   $1,234.33   $1,234.35   $1,234.40