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 $18.84   11.853%   $21.0731   $21.07   $21.05   $21.10  
  Purchase #2 $67.56   10.729%   $74.8085   $74.81   $74.80   $74.80  
  Purchase #3 $136.90   12.863%   $154.5094   $154.51   $154.50   $154.50  
  Purchase #4 $99.61   12.597%   $112.1579   $112.16   $112.15   $112.20  
  Purchase #5 $143.94   9.743%   $157.9641   $157.96   $157.95   $158.00  
  Purchase #6 $250.43   10.859%   $277.6242   $277.62   $277.60   $277.60  
  Purchase #7 $122.58   8.587%   $133.1059   $133.11   $133.10   $133.10  
  7 purchase totals = $839.86       $931.2431   $931.24   $931.15   $931.30