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 $204.51   13.456%   $232.0289   $232.03   $232.05   $232.00  
  Purchase #2 $110.95   11.186%   $123.3609   $123.36   $123.35   $123.40  
  Purchase #3 $185.99   6.306%   $197.7185   $197.72   $197.70   $197.70  
  Purchase #4 $102.77   13.815%   $116.9677   $116.97   $116.95   $117.00  
  Purchase #5 $92.25   3.857%   $95.8081   $95.81   $95.80   $95.80  
  Purchase #6 $20.61   9.304%   $22.5276   $22.53   $22.55   $22.50  
  Purchase #7 $185.27   9.646%   $203.1411   $203.14   $203.15   $203.10  
  7 purchase totals = $902.35       $991.5528   $991.56   $991.55   $991.50