You might get a 'mix' of replies to your post. One school says to lay plastic if the home is a sieve for leaking or drawing air. Assuming the attic is UNHEATED, the problem is with warm air meeting cool air where the plastic is laid. You get condensation, then mold and mildew, then possibly dripping water, then maybe even an allergy problem.
The other school says that if the house is very tight to begin with, and leakage to the attic is, basically, non-existant, lay down the plastic. I guess the risks are less in the latter case but still has risks, nonetheless.
Personally, my home is very tight. (I know; I sheathed and insulated it.) There is still some leakage from outside but I know where the leaks are. I have cellulose insulation in the attic. It's been blown such that it's ABOVE the attic joists. Fiberglass insulation is more 'concerned' with 'height' because it's usually attached to the joists (but doesn't really have to be because gravity keeps it in place.) The cellulose is MEANT to be 'luffed' so don't compress it. If you're gonna store things up there, I'd go w/fiberglass. Get the proper R-value depending if it's heated or not. (I'm gonna assume it's unheated because you didn't say anything about the attic roof being insulated.)