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 $227.51   11.611%   $253.9262   $253.93   $253.95   $253.90  
  Purchase #2 $92.42   10.864%   $102.4605   $102.46   $102.45   $102.50  
  Purchase #3 $168.61   3.548%   $174.5923   $174.59   $174.60   $174.60  
  Purchase #4 $35.68   7.756%   $38.4473   $38.45   $38.45   $38.40  
  Purchase #5 $165.38   10.588%   $182.8904   $182.89   $182.90   $182.90  
  Purchase #6 $47.52   11.643%   $53.0528   $53.05   $53.05   $53.10  
  Purchase #7 $177.35   8.617%   $192.6322   $192.63   $192.65   $192.60  
  7 purchase totals = $914.47       $998.0017   $998.00   $998.05   $998.00