In his nic storage thread, @Rossum says that nic will not expand in the freezer, whether in PG or VG. If you remove it from the freezer on a day that is warmer than when you put it in, however, it may expand, and liquid may force its way through the seal on the caps as it warms up.
He goes on to say, "Still, a little headroom is good. With zero headroom in the bottle, you'll have something close to a hard vacuum in the headroom that will develop as the liquid contracts at freezer temperature. That means a pressure differential of 14.7 psi at sea level. Most reasonable size glass bottles 500 ml or smaller can withstand that, but why put undue mechanical stress on them?"
