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 $117.68   10.827%   $130.4212   $130.42   $130.40   $130.40  
  Purchase #2 $129.53   11.383%   $144.2744   $144.27   $144.25   $144.30  
  Purchase #3 $115.63   7.271%   $124.0375   $124.04   $124.05   $124.00  
  Purchase #4 $188.65   10.296%   $208.0734   $208.07   $208.05   $208.10  
  Purchase #5 $114.33   12.621%   $128.7596   $128.76   $128.75   $128.80  
  Purchase #6 $143.31   12.291%   $160.9242   $160.92   $160.90   $160.90  
  Purchase #7 $84.97   9.356%   $92.9198   $92.92   $92.90   $92.90  
  7 purchase totals = $894.10       $989.4101   $989.40   $989.30   $989.40