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 $15.80   10.252%   $17.4198   $17.42   $17.40   $17.40  
  Purchase #2 $122.50   10.800%   $135.73   $135.73   $135.75   $135.70  
  Purchase #3 $20.31   5.427%   $21.4122   $21.41   $21.40   $21.40  
  Purchase #4 $222.29   13.373%   $252.0168   $252.02   $252.00   $252.00  
  Purchase #5 $117.79   7.455%   $126.5712   $126.57   $126.55   $126.60  
  Purchase #6 $184.57   3.369%   $190.7882   $190.79   $190.80   $190.80  
  Purchase #7 $111.57   3.487%   $115.4604   $115.46   $115.45   $115.50  
  7 purchase totals = $794.83       $859.3986   $859.40   $859.35   $859.40