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 $227.30   13.827%   $258.7288   $258.73   $258.75   $258.70  
  Purchase #2 $48.50   6.412%   $51.6098   $51.61   $51.60   $51.60  
  Purchase #3 $222.49   7.627%   $239.4593   $239.46   $239.45   $239.50  
  Purchase #4 $150.28   4.780%   $157.4634   $157.46   $157.45   $157.50  
  Purchase #5 $15.57   3.610%   $16.1321   $16.13   $16.15   $16.10  
  Purchase #6 $104.93   5.597%   $110.8029   $110.80   $110.80   $110.80  
  Purchase #7 $130.83   9.203%   $142.8703   $142.87   $142.85   $142.90  
  7 purchase totals = $899.90       $977.0666   $977.06   $977.05   $977.10