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 $131.61   5.820%   $139.2697   $139.27   $139.25   $139.30  
  Purchase #2 $62.37   13.240%   $70.6278   $70.63   $70.65   $70.60  
  Purchase #3 $109.65   6.611%   $116.899   $116.90   $116.90   $116.90  
  Purchase #4 $222.68   6.712%   $237.6263   $237.63   $237.65   $237.60  
  Purchase #5 $184.61   12.643%   $207.9502   $207.95   $207.95   $208.00  
  Purchase #6 $55.51   6.649%   $59.2009   $59.20   $59.20   $59.20  
  Purchase #7 $21.34   13.311%   $24.1806   $24.18   $24.20   $24.20  
  7 purchase totals = $787.77       $855.7545   $855.76   $855.80   $855.80