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 $248.68   12.827%   $280.5782   $280.58   $280.60   $280.60  
  Purchase #2 $76.87   7.520%   $82.6506   $82.65   $82.65   $82.70  
  Purchase #3 $96.88   4.714%   $101.4469   $101.45   $101.45   $101.40  
  Purchase #4 $208.64   5.537%   $220.1924   $220.19   $220.20   $220.20  
  Purchase #5 $139.69   13.251%   $158.2003   $158.20   $158.20   $158.20  
  Purchase #6 $224.39   6.286%   $238.4952   $238.50   $238.50   $238.50  
  Purchase #7 $19.38   11.561%   $21.6205   $21.62   $21.60   $21.60  
  7 purchase totals = $1,014.53       $1,103.1841   $1,103.19   $1,103.20   $1,103.20