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 $237.75   13.681%   $270.2766   $270.28   $270.30   $270.30  
  Purchase #2 $120.51   3.674%   $124.9375   $124.94   $124.95   $124.90  
  Purchase #3 $208.37   10.123%   $229.4633   $229.46   $229.45   $229.50  
  Purchase #4 $217.53   10.342%   $240.027   $240.03   $240.05   $240.00  
  Purchase #5 $48.33   10.855%   $53.5762   $53.58   $53.60   $53.60  
  Purchase #6 $74.97   8.335%   $81.2187   $81.22   $81.20   $81.20  
  Purchase #7 $234.85   11.730%   $262.3979   $262.40   $262.40   $262.40  
  7 purchase totals = $1,142.31       $1,261.8972   $1,261.91   $1,261.95   $1,261.90