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 $28.47   6.789%   $30.4028   $30.40   $30.40   $30.40  
  Purchase #2 $92.43   11.852%   $103.3848   $103.38   $103.40   $103.40  
  Purchase #3 $29.84   4.110%   $31.0664   $31.07   $31.05   $31.10  
  Purchase #4 $245.41   9.176%   $267.9288   $267.93   $267.95   $267.90  
  Purchase #5 $248.71   11.222%   $276.6202   $276.62   $276.60   $276.60  
  Purchase #6 $81.83   10.434%   $90.3681   $90.37   $90.35   $90.40  
  Purchase #7 $16.65   4.855%   $17.4584   $17.46   $17.45   $17.50  
  7 purchase totals = $743.34       $817.2295   $817.23   $817.20   $817.30