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 $17.62   3.488%   $18.2346   $18.23   $18.25   $18.20  
  Purchase #2 $56.94   13.730%   $64.7579   $64.76   $64.75   $64.80  
  Purchase #3 $67.89   10.786%   $75.2126   $75.21   $75.20   $75.20  
  Purchase #4 $114.47   10.434%   $126.4138   $126.41   $126.40   $126.40  
  Purchase #5 $178.48   12.502%   $200.7936   $200.79   $200.80   $200.80  
  Purchase #6 $13.80   7.271%   $14.8034   $14.80   $14.80   $14.80  
  Purchase #7 $81.59   9.357%   $89.2244   $89.22   $89.20   $89.20  
  7 purchase totals = $530.79       $589.4403   $589.42   $589.40   $589.40