For starters, i'd leave all the tops on while curing. Cutting anything off can lead to infection that will keep your onions from storing well. If you really want to cut some for the experiment, go easy and be sure to eat those onions first.

As far as the rain. Persistent moisture while onions are finishing up can cause splitting and various mold/mildew issues. One soaking may not be a huge problem as onions keep sizing up from when the start to fall till they are all down, which usually takes a few weeks. Some moisture at this time is helpful.
If they are really wet or more rain is predicted, you could pull one every day to check for problems. Then pull them all if necessary.
-Mark

THANK You so much Mark! I pulled a couple for dinner tonight, the kitchen drawer was out. Mine are not as large as yours, but DH gave me a big thumbs up for flavor.

Um what kind of problems might I see? ( other than a 210 lb onion eater :) Only my second year growing onions...

Mostly watch for basal rot at the root plate and any core rot (black stuff inside the onion or at the neck). Also you could get some surface mold issues on the outer skin.

As i'm sure you're used to what a normal onion looks like inside and out, t'll be pretty obvious.