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 $208.64   12.492%   $234.7033   $234.70   $234.70   $234.70  
  Purchase #2 $15.80   8.559%   $17.1523   $17.15   $17.15   $17.20  
  Purchase #3 $120.45   12.474%   $135.4749   $135.47   $135.45   $135.50  
  Purchase #4 $104.93   12.105%   $117.6318   $117.63   $117.65   $117.60  
  Purchase #5 $160.85   13.834%   $183.102   $183.10   $183.10   $183.10  
  Purchase #6 $59.30   8.781%   $64.5071   $64.51   $64.50   $64.50  
  Purchase #7 $70.41   10.867%   $78.0615   $78.06   $78.05   $78.10  
  7 purchase totals = $740.38       $830.6329   $830.62   $830.60   $830.70