Re-open fill plug to check level after transaxle fluid change?

If you've put four quarts of ATF WS in, and there are no leaks after you've completed the job, then I would say you can rest and congratulate yourself on a job well-done.

 

When I changed mine, I could not even get 4 full quarts in before it was running out the fill hole with the car level. I would estimate that it was actually about 3.7 or 3.8 quarts to refill.

I think rechecking the level is overkill because of the reason you state: the method of checking (finger in the fill hole) is relatively insensitive to changes in total volume in the transaxle.

 

When I had my '04 Prius, I changed the transaxle fluid 5 times. I NEVER had the fluid go down after driving and rechecking about a week later

In something like a differential, the fluid upon being drained and refilled, you may notice you can add a bit more after waiting a week or so

For example, the rear axle on my FJ. The fill plug level is such that a bit of gear oil will be in the axle tubes. Drain the axle, and refill until the fluid seeps out the fill hole.

Drive around for a week or so, recheck cold, and I can perhaps squeeze another 1.5 cups of gear oil to where it seeps out again. Not enough to make any difference long term

And that certainly doesn't apply to a Prius

As long as you had the Prius perfectly level while refilling, you should be fine. If you had the front end up on ramps, due to the location of the fill hole, the transaxle will be overfilled a bit

 

I just changed my tranny fluid yesterday. I had the car sitting "as level as possible" in the garage. I could barely get to the fill hole, and certaily could not get my figure in the hole. My fluid pump just barely fit under the car in front of the engine. I pumped a bit more than 3.8 quarts into the tranny until fluid started seriously draining out of the fill hole. I waited for the flow to diminish to very intermittant dribbles (meaning I didn't think that even 1/2 oz more of fluid was going to dribble out), and then I tightened the fill plug to 29 ft-lbs. So far, the tranny is working great.

With typical automatic transmissions, there is a fluid filter and various fluid "circuits" that can trap air bubbles. After changing automatic trans fluid, the car must be driven a bit with the gear selector positioned in each gear available (typically, P, R, N, D, 2, 1) to flush out the air. The fluid must then be checked again--it will be much less than full. Top up the fluid level and it's done.

But as far as I know, the Prius tranny doesn't have specific fluid circuits, and thus doesn't need to be driven and immediately rechecked like an automatic tranny.