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 $49.78   13.138%   $56.3201   $56.32   $56.30   $56.30  
  Purchase #2 $125.44   4.482%   $131.0622   $131.06   $131.05   $131.10  
  Purchase #3 $56.76   12.704%   $63.9708   $63.97   $63.95   $64.00  
  Purchase #4 $53.26   5.118%   $55.9858   $55.99   $56.00   $56.00  
  Purchase #5 $228.76   10.278%   $252.272   $252.27   $252.25   $252.30  
  Purchase #6 $243.44   9.276%   $266.0215   $266.02   $266.00   $266.00  
  Purchase #7 $102.92   9.641%   $112.8425   $112.84   $112.85   $112.80  
  7 purchase totals = $860.36       $938.4749   $938.47   $938.40   $938.50