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 $133.99   3.600%   $138.8136   $138.81   $138.80   $138.80  
  Purchase #2 $208.68   13.369%   $236.5784   $236.58   $236.60   $236.60  
  Purchase #3 $74.35   12.331%   $83.5181   $83.52   $83.50   $83.50  
  Purchase #4 $232.50   11.860%   $260.0745   $260.07   $260.05   $260.10  
  Purchase #5 $234.96   9.451%   $257.1661   $257.17   $257.15   $257.20  
  Purchase #6 $217.97   9.262%   $238.1584   $238.16   $238.15   $238.20  
  Purchase #7 $71.36   5.595%   $75.3526   $75.35   $75.35   $75.40  
  7 purchase totals = $1,173.81       $1,289.6617   $1,289.66   $1,289.60   $1,289.80