We are traveling to Venice in April. Do we need guided tours for any of the popular attractions?

True, you don't need guided tours. As for getting a vaporetto pass (you can get 1, 2, 3, or 7 day passes, as needed), that's a good idea - without one, each ride on a vaporetto is 7.50 euros. Places you can't get to on foot, but can by vaporetto, include the islands in the Venetian lagoon (Murano, Burano, Torcello, etc.), and San Giorgio Maggiore.

We were in Venice recently (7 nights) and did not need a guided tour. We avoided the busy San Marco area mid day. We saw St. Mark's early am. We arrived at opening & were 1st in line. We walked off the beaten path neighborhoods. We got the 3 day Vaporetto pass the last few days. We went to San Giorgio Maggiore for the exquisite view looking back at Venice. We also took the Vaporetto to Burano, Torcello and Lido (film festival.) Enjoy wandering for unexpected surprises!

Getting the three day vaporetto pass is a good idea, if you're there for that amount of time. It's more cost effective than purchasing one time tickets each time you want to ride. Also you can use the vaporetto to go out to Burano and Murano. (if you have more than one day there) And a trip down the Grand Canal is also a must do. Try to get the front seats for a memorable ride.
As for the sites, it depends on what you want to see. You can wait in line to get tickets to see the Doge's palace and do a self guided tour from room to room, but the Secret Itineraries guided tour was worth getting tickets in advance, and with advance tickets there was NO waiting, other than for the tour to start.
At st. marks, You can wait in line to get into St. Mark's Basilica and walk around on your own, but you will pay extra to see the Golden Pali behind the altar or pay extra to climb up to the terrace, or you can book a tour in advance that includes all these things, so you don't have to wait in line and you get a guided tour as well. I've done both. Entering on my own and just walking around was wasted time for me as I had no idea what I was seeing or the significance. The guided tour brought everything to life and we got to see other areas that are not open to the public.
Most of the other sites are open: St. Mark's square, walking around the narrow streets of Venice, crossing on the Rialto Bridge, viewing the Bridge of Sighs, etc. are all free (so far) and open.
We took one of the free (they work on tips) walking tours and enjoyed that a lot. It took us through some of the neighborhoods we never would have walked through but for the walking tour.

We are going to Venice with our teenaged granddaughters this summer for 5 nights. We plan to buy the 3 day pass for young people called Rolling Venice for them. http://www.veneziaunica.it/en/content/rolling-venice

EDiT: for young people the 40€ 3day pass should cost 6€ + 22€ for three days =28€

hey db2brant
you don't need a guided tour, read europeforvisitors.com with lots of info. it will be busy and crowded. how many days will you be there? plan early morning and late afternoon outings to popular places. cruise passengers, daytrippers cram into them. walk the back canals and alleys away from them, take early boat to the islands, buy the vaporetto pass if there for 3 days.
lots of stairs/steps and bridges to cross, you're on an adventure get lost and find your way zigging and zagging everywhere. if you are prepared for all this you will enjoy your time, it's a beautiful place. been there 3 times. interested in doing a cicchetti crawl (destination360.com search cicchetti tour). pub crawl with appetizers and learning about venice with alessandro (schezzini.it) if doing a gondola ride, use one from back canals, 80E per boat. the grand canal is very busy with all boats using it.
i sat at a restaurant outside table across train station drinking prosecco, "best tiramisu" and people watching, give yourself time for that. there is a crepe place "cocaeta" off fondementa de canareggio (near stop crea), enjoy
aloha

You can get reservation tickets €3,00 for St. Mark's Basilica to skip the line. There are days when the line to get in can take upwards of an hour. The skip the line door is to the left of the main entrance.

https://www.venetoinside.com/attraction-tickets-in-veneto/tickets/skip-the-line-saint-marks-basilica/

You need to print the voucher, and bring it with you. Must be a paper copy.

Try to schedule your reservation around 11:30-12:00 so you can see the Moasics lit.

The only attraction I would book a tour is the Doge Palace. It too can be very busy.