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 $45.59   12.266%   $51.1821   $51.18   $51.20   $51.20  
  Purchase #2 $237.40   6.529%   $252.8998   $252.90   $252.90   $252.90  
  Purchase #3 $65.25   12.805%   $73.6053   $73.61   $73.60   $73.60  
  Purchase #4 $117.43   11.448%   $130.8734   $130.87   $130.85   $130.90  
  Purchase #5 $50.48   12.135%   $56.6057   $56.61   $56.60   $56.60  
  Purchase #6 $146.77   7.245%   $157.4035   $157.40   $157.40   $157.40  
  Purchase #7 $53.73   12.872%   $60.6461   $60.65   $60.65   $60.60  
  7 purchase totals = $716.65       $783.2159   $783.22   $783.20   $783.20