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 $65.74   11.461%   $73.2745   $73.27   $73.25   $73.30  
  Purchase #2 $20.81   4.462%   $21.7385   $21.74   $21.75   $21.70  
  Purchase #3 $244.81   3.729%   $253.939   $253.94   $253.95   $253.90  
  Purchase #4 $12.93   3.271%   $13.3529   $13.35   $13.35   $13.40  
  Purchase #5 $60.53   4.251%   $63.1031   $63.10   $63.10   $63.10  
  Purchase #6 $163.60   9.244%   $178.7232   $178.72   $178.70   $178.70  
  Purchase #7 $209.68   3.609%   $217.2474   $217.25   $217.25   $217.20  
  7 purchase totals = $778.10       $821.3786   $821.37   $821.35   $821.30