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 $18.48   13.551%   $20.9842   $20.98   $21.00   $21.00  
  Purchase #2 $174.40   11.111%   $193.7776   $193.78   $193.80   $193.80  
  Purchase #3 $248.81   10.324%   $274.4971   $274.50   $274.50   $274.50  
  Purchase #4 $25.81   9.429%   $28.2436   $28.24   $28.25   $28.20  
  Purchase #5 $244.66   12.397%   $274.9905   $274.99   $275.00   $275.00  
  Purchase #6 $19.69   9.730%   $21.6058   $21.61   $21.60   $21.60  
  Purchase #7 $140.74   7.617%   $151.4602   $151.46   $151.45   $151.50  
  7 purchase totals = $872.59       $965.5590   $965.56   $965.60   $965.60