It depends on what airline and what time you can get flights departing from Venice. There are more non-stop flights from Rome to the US than there are from Venice. This means you can usually find later flights from Rome since you don't need to connect in Europe. Many Venice flights depart very early (6am). There are a few non-stops to the US from Venice (Delta has a seasonal VCE/JFK), but it depends which airline you are using.

If I'm flying to Europe, I schedule as many days as possible. If I'm spending all that time and money on a flight, might as well stay awhile.

Try typing an open jaws (multi-location) request into Kayak and see what a proposed flight schedule would look like. I believe that the only direct flights between Washington D.C. and Italy are into/out of Rome on United. Coming home usually
involves an overnight at the airport in Rome or in Mestre for
a Venice departure, as I remember. Have a wonderful trip.

Pat

Personally I would fly out from Venice with the option of changing planes in Paris, Amsterdam, London or Frankfurt (depending on airline) than catching a train all the way back to Rome. If you fly out of Venice your luggage will be checked through to the final destination. Plus you'll have extra time in Venice

Ultimately depends on cost and convenience for you. Have you booked your flights to Rome? If not search kayak.com for multi destinations flights to see what options it suggests.

We've done the Venice, CDG, IAD route and, if you can avoid connecting through CDG, I would. I don't think the signage within CDG is very clear (and I know French) and going back through security is unusually chaotic. I've found Frankfort or Amsterdam or even London easier.

In the past, the DC flights out of Venice were some of the earliest so you'd lose that day. If you could get a later flight out of Rome, you could be a tourist a bit longer in Venice.

Ideally, flying into Venice and out of Rome would be the most convenient if you are not early risers. Hub and I flew out of Venice several times and never had a problem getting there via VAP to bus or VAP to taxi in the early hours.

I don't recall any non-stops to Venice or Rome from DC. Am I wrong now? There was one to Paris and one to Milan out of Dulles.

Non-stop flights from the US are mostly to Rome or Milan. There are a few/seasonal flights to Venice and Pisa (Delta - JFK-PSA).

For us, flying into Rome and out of Venice is what made the most sense.

I have the most energy at the beginning of a trip, and I think that fits Rome better.

Also, leaving Venice at 8am will let us get back home early enough to hug our kids and put them to bed.

Just remember that an 8am flight departure from Venice, if you are staying IN Venice, will mean a very early start check-out, to get to the mainland, transfer to the airport, check-in, etc. I think that's why a lot of people suggest arrive in Venice, depart from Rome.
Another option would be to stay near the Venice airport the night before your departure but that requires another change of hotel.

Although hollywoodsc is correct, early a.m. transportation via night VAP is more than adequate to get one to the Piazzale Roma and the bus or taxi to airport. It's just the early part that can be daunting to some.

We stayed around Campo Stefano. Got up at 3:40 a.m., walked to catch 4:15 night VAP to Piazzalle Roma and took a taxi from there. We were at airport by 5 a.m. and actually arrived before the airline staff. (VAP ticket could be used on the bus if I'm remembering correctly.) We flew to Dulles via Paris. My notes don't say what time we actually arrived at our Md. home but I'm sure it was late afternoon or early evening.